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

site_scons/commandline.scons

@@ -63,6 +63,11 @@ vars.AddVariables(
         help="Enable debug build",
         default=True,
     ),
+    BoolVariable(
+        "LIB_DEBUG",
+        help="Enable debug build for libraries",
+        default=False,
+    ),
     BoolVariable(
         "COMPACT",
         help="Optimize for size",