avr: configure emulator in board files

Instead of specifying the emulator command in atmega328p.json, specify
it in the two boards based on it (arduino and arduino-nano). This makes
the configuration consistent with the machine package, which only
defines the CPUFrequency function in the board files (and not in
machine_atmega328p.json).

targets/atmega328p.json

@@ -12,6 +12,5 @@
 	"extra-files": [
 		"targets/avr.S",
 		"src/device/avr/atmega328p.s"
-	],
-	"emulator": ["simavr", "-m", "atmega328p", "-f", "16000000"]
+	]
 }