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/environ.scons

@@ -61,8 +61,8 @@ coreenv = VAR_ENV.Clone(
     ABSPATHGETTERFUNC=extract_abs_dir_path,
     # Setting up temp file parameters - to overcome command line length limits
     TEMPFILEARGESCFUNC=tempfile_arg_esc_func,
-    FBT_SCRIPT_DIR=Dir("#/scripts"),
     ROOT_DIR=Dir("#"),
+    FBT_SCRIPT_DIR="${ROOT_DIR}/scripts",
 )
 
 # If DIST_SUFFIX is set in environment, is has precedence (set by CI)