[FL-2859,2838] fbt: improvements for FAPs (#1813)

* fbt: assets builder for apps WIP
* fbt: automatically building private fap assets
* docs: details on how to use image assets
* fbt: renamed fap_assets -> fap_icons
* fbt: support for fap_extbuild field
* docs: info on fap_extbuild
* fbt: added --proxy-env parame ter
* fbt: made firmware_cdb & updater_cdb targets always available
* fbt: renamed fap_icons -> fap_icon_assets
* fbt: deprecated firmware_* target names for faps; new alias is "fap_APPID"
* fbt: changed intermediate file locations for external apps
* fbt: support for fap_private_libs; docs: updates
* restored mbedtls as global lib
* scripts: lint.py: skip "lib" subfolder
* fbt: Sanity checks for building advanced faps as part of fw
* docs: info on fap_private_libs; fbt: optimized *.fam indexing
* fbt: cleanup; samples: added sample_icons app
* fbt: moved example app to applications/examples
* linter fix
* docs: readme fixes
* added applications/examples/application.fam stub
* docs: more info on private libs

Co-authored-by: あく <alleteam@gmail.com>

site_scons/environ.scons

@@ -12,14 +12,20 @@ forward_os_env = {
     "PATH": os.environ["PATH"],
 }
 # Proxying CI environment to child processes & scripts
-for env_value_name in (
+variables_to_forward = [
     "WORKFLOW_BRANCH_OR_TAG",
     "DIST_SUFFIX",
     "HOME",
     "APPDATA",
     "PYTHONHOME",
     "PYTHONNOUSERSITE",
-):
+    "TMP",
+    "TEMP",
+]
+if proxy_env := GetOption("proxy_env"):
+    variables_to_forward.extend(proxy_env.split(","))
+
+for env_value_name in variables_to_forward:
     if environ_value := os.environ.get(env_value_name, None):
         forward_os_env[env_value_name] = environ_value