[FL-2859,2838] fbt: improvements for FAPs (#1813)

* fbt: assets builder for apps WIP
* fbt: automatically building private fap assets
* docs: details on how to use image assets
* fbt: renamed fap_assets -> fap_icons
* fbt: support for fap_extbuild field
* docs: info on fap_extbuild
* fbt: added --proxy-env parame ter
* fbt: made firmware_cdb & updater_cdb targets always available
* fbt: renamed fap_icons -> fap_icon_assets
* fbt: deprecated firmware_* target names for faps; new alias is "fap_APPID"
* fbt: changed intermediate file locations for external apps
* fbt: support for fap_private_libs; docs: updates
* restored mbedtls as global lib
* scripts: lint.py: skip "lib" subfolder
* fbt: Sanity checks for building advanced faps as part of fw
* docs: info on fap_private_libs; fbt: optimized *.fam indexing
* fbt: cleanup; samples: added sample_icons app
* fbt: moved example app to applications/examples
* linter fix
* docs: readme fixes
* added applications/examples/application.fam stub
* docs: more info on private libs

Co-authored-by: あく <alleteam@gmail.com>

documentation/AppsOnSDCard.md

@@ -2,7 +2,7 @@
 
 [fbt](./fbt.md) has support for building applications as FAP files. FAP are essentially .elf executables with extra metadata and resources bundled in.
 
-FAPs are built with `firmware_extapps` (or `plugin_dist`) **`fbt`** targets.
+FAPs are built with `faps` **`fbt`** target. They can also be deployed to `dist` folder with `plugin_dist` **`fbt`** target.
 
 FAPs do not depend on being run on a specific firmware version. Compatibility is determined by the FAP's metadata, which includes the required [API version](#api-versioning).
 
@@ -18,6 +18,17 @@ To build your application as a FAP, just create a folder with your app's source
  * To build all FAPs, run `./fbt plugin_dist`.
 
 
+## FAP assets
+
+FAPs can include static and animated images as private assets. They will be automatically compiled alongside application sources and can be referenced the same way as assets from the main firmware.
+
+To use that feature, put your images in a subfolder inside your application's folder, then reference that folder in your application's manifest in `fap_icon_assets` field. See [Application Manifests](./AppManifests.md#application-definition) for more details.
+
+To use these assets in your application, put `#include "{APPID}_icons.h"` in your application's source code, where `{APPID}` is the `appid` value field from your application's manifest. Then you can use all icons from your application's assets the same way as if they were a part of `assets_icons.h` of the main firmware.
+
+Images and animated icons must follow the same [naming convention](../assets/ReadMe.md#asset-naming-rules) as those from the main firmware.
+
+
 ## Debugging FAPs
 
 **`fbt`** includes a script for gdb-py to provide debugging support for FAPs, `debug/flipperapps.py`. It is loaded in default debugging configurations by **`fbt`** and stock VSCode configurations.
@@ -53,13 +64,13 @@ App loader allocates memory for the application and copies it to RAM, processing
 
 Not all parts of firmware are available for external applications. A subset of available functions and variables is defined in "api_symbols.csv" file, which is a part of firmware target definition in `firmware/targets/` directory. 
 
-**`fbt`** uses semantic versioning for API versioning. Major version is incremented when there are breaking changes in the API, minor version is incremented when there are new features added. 
+**`fbt`** uses semantic versioning for API. Major version is incremented when there are breaking changes in the API, minor version is incremented when new features are added. 
 
 Breaking changes include:
 - removal of a function or a global variable;
 - changing the signature of a function.
 
-API versioning is mostly automated by **`fbt`**. When rebuilding the firmware, **`fbt`** checks if there are any changes in the API exposed by headers gathered from `SDK_HEADERS`. If there are, it stops the build, adjusts the API version and asks the user to go through the changes in .csv file. New entries are marked with "`?`" mark, and the user is supposed to change the mark to "`+`" for the entry to be exposed for FAPs, "`-`" for it to be unavailable.
+API versioning is mostly automated by **`fbt`**. When rebuilding the firmware, **`fbt`** checks if there are any changes in the API exposed by headers gathered from `SDK_HEADERS`. If so, it stops the build, adjusts the API version and asks the user to go through the changes in .csv file. New entries are marked with "`?`" mark, and the user is supposed to change the mark to "`+`" for the entry to be exposed for FAPs, "`-`" for it to be unavailable.
 
 **`fbt`** will not allow building a firmware until all "`?`" entries are changed to "`+`" or "`-`".