fbt: 'target' field for apps; lib debugging support (#1995)

* fbt: added 'target' field to application manifest
* fbt: earlier pagination setup for gdb
* fbt: added LIB_DEBUG flag
* fbt: sdk: added SDK_MAP_FILE_SUBST

SConstruct

@@ -7,6 +7,7 @@
 # construction of certain targets behind command-line options.
 
 import os
+from fbt.util import path_as_posix
 
 DefaultEnvironment(tools=[])
 
@@ -200,9 +201,7 @@ firmware_debug = distenv.PhonyTarget(
     source=firmware_env["FW_ELF"],
     GDBOPTS="${GDBOPTS_BASE}",
     GDBREMOTE="${OPENOCD_GDB_PIPE}",
-    FBT_FAP_DEBUG_ELF_ROOT=firmware_env.subst("$FBT_FAP_DEBUG_ELF_ROOT").replace(
-        "\\", "/"
-    ),
+    FBT_FAP_DEBUG_ELF_ROOT=path_as_posix(firmware_env.subst("$FBT_FAP_DEBUG_ELF_ROOT")),
 )
 distenv.Depends(firmware_debug, firmware_flash)
 
@@ -212,9 +211,7 @@ distenv.PhonyTarget(
     source=firmware_env["FW_ELF"],
     GDBOPTS="${GDBOPTS_BASE} ${GDBOPTS_BLACKMAGIC}",
     GDBREMOTE="${BLACKMAGIC_ADDR}",
-    FBT_FAP_DEBUG_ELF_ROOT=firmware_env.subst("$FBT_FAP_DEBUG_ELF_ROOT").replace(
-        "\\", "/"
-    ),
+    FBT_FAP_DEBUG_ELF_ROOT=path_as_posix(firmware_env.subst("$FBT_FAP_DEBUG_ELF_ROOT")),
 )
 
 # Debug alien elf