Add a minimal implementation of `strerror()`.
All other targets use either newlib or picolibc and provide this
function, add it to AVR so we can rely on it in common code.
Split atmega_common code into avr8_common folder. This moves common
avr8 code to be used for all avr8 variants: tiny, mega and xmega.
Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>