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).
It was `avr-atmel-none`, which is incorrect. It must be
`avr-unknown-unknown`.
Additionally, there is no reason to specify the target triple per chip,
it can be done for all AVR chips at once as it doesn't vary like
Cortex-M chips.
It does not appear to be necessary for these devices but might result in
more appropriate libraries to be linked in.
It is best to _not_ specify the exact MCU because otherwise a few other
settings (such as startfiles and some linker script configs) also get
set, which we do manually anyway and should not be interfered with.
I discovered this while working on support for the atmega1284.
