fbt: fixes for ufbt pt3 (#1970)

* fbt: replaced debug dir paths with FBT_DEBUG_DIR
* scripts: updated requirements.txt
* fbt: fixed wrong import
* fbt: removed delayed import for file2image
* fbt: added UPDATE_BUNDLE_DIR internal var
* fbt: cleaner internal management of extapps
* applications: added fap_libs for core apps to link with resources when building with --extra-ext-apps
* fbt: removed deprecation stub for faps
* fbt: added quotation for icons build cmd
* fbt: reworked BUILD_DIR & fap work dir handling; fap debug: using debug elf path from fbt
* fbt: explicit LIB_DIST_DIR

site_scons/commandline.scons

@@ -147,7 +147,7 @@ vars.AddVariables(
     PathVariable(
         "SVD_FILE",
         help="Path to SVD file",
-        validator=PathVariable.PathIsFile,
+        validator=PathVariable.PathAccept,
         default="",
     ),
     PathVariable(