fbt: support for LOADER_AUTOSTART; post-build size stats (#1594)

* fbt: restored LOADER_AUTOSTART support
* scripts: added fwsize.py wrapper for size command; fbt: changed size post-build stats to fwsize.py call
* fbt: removed size wrapper
* fbt: added stats for binary flash size in pages
* fbt: hint on build options details
* scripts: fixed fwsize.py for *nix

site_scons/commandline.scons

@@ -174,6 +174,12 @@ vars.Add(
     default="update_default",
 )
 
+vars.Add(
+    "LOADER_AUTOSTART",
+    help="Application name to automatically run on Flipper boot",
+    default="",
+)
+
 
 vars.Add(
     "FIRMWARE_APPS",