avr: add support for the digispark

Blinking the on-board LED works. Nothing else has been tested yet.

targets/digispark.json

@@ -0,0 +1,17 @@
+{
+	"inherits": ["avr"],
+	"llvm-target": "avr-atmel-none",
+	"build-tags": ["digispark", "attiny85", "attiny", "avr2", "avr25"],
+	"pre-link-args": [
+		"-nostartfiles",
+		"-mmcu=attiny85",
+		"-Wl,--defsym=_bootloader_size=2180",
+		"-Wl,--defsym=_stack_size=128",
+		"-T", "src/device/avr/attiny85.ld",
+		"-T", "targets/avr.ld",
+		"-Wl,--gc-sections",
+		"targets/avr.S",
+		"src/device/avr/attiny85.s"
+	],
+	"flash": "micronucleus --run {hex}"
+}