[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/commandline.scons

@@ -34,6 +34,14 @@ AddOption(
     help="List of applications to forcefully build as standalone .elf",
 )
 
+AddOption(
+    "--proxy-env",
+    action="store",
+    dest="proxy_env",
+    default="",
+    help="Comma-separated list of additional environment variables to pass to child SCons processes",
+)
+
 
 # Construction environment variables
 
@@ -230,6 +238,7 @@ vars.Add(
         ("applications/system", False),
         ("applications/debug", False),
         ("applications/plugins", False),
+        ("applications/examples", False),
         ("applications_user", False),
     ],
 )