Flipper Zero Application build utils

Typically for usage from build script.

It do following things:

1. Build FAP manifest using fam crate
2. Read env var FLIPPER_SDK_PATH_ENV
3. Copy or link (now just link) the output dir where manifest (fam) is to $FLIPPER_FW_SRC_PATH/applications_user/{fap-id}/
4. Same for assets such as icons

After this you can just execute ./fbt firmware_{fap-id} from the root flipperzero-firmware repository clone.

$FLIPPER_FW_SRC_PATH should points to working copy of the flipperzero-firmware repository.

Example.