Support Digistump/Digispark dtiny AVR core for digispark-tiny

[zackees]

Follow-up from FastLED/FastLED#2387.

FastLED now routes all board builds through fbuild with no board whitelist or PlatformIO fallback path. The remaining compatibility issue from FastLED/FastLED#2387 should be fixed here in fbuild.

Observed local behavior after generating the FastLED .build/pio/digispark-tiny project:

fbuild .build\pio\digispark-tiny build -e digispark-tiny --dry-run --verbose
Environment: digispark-tiny
Daemon is running. Dry-run complete.

The actual build fails before compilation:

fbuild .build\pio\digispark-tiny build -e digispark-tiny --verbose
BUILDING digispark-tiny
Board: Digispark USB / ATTINY85 @ 16MHz
Memory: 5.87KB Flash, 512 bytes RAM
Toolchain: avr-gcc 7.3.0
build error: config error: no AVR framework registered for core 'dtiny' (available: [MegaCoreX, tinymodern, tiny, MiniCore, arduino, arduino_megaavr, megatinycore])

Ask: add the framework/core mapping needed for the Digistump/Digispark dtiny AVR core so digispark-tiny can build through fbuild.