[AVR] Split startup code into different .init sections according to avr-libs convention.
Split startup code into separate procedures that aligns more or less with avr-libc's .init sections. This allows a user for example to run a RAM test before the RTL copies initialized data into RAM. Removed unused directives and defines. Moved all common code into avrcommon.inc and deleted start.inc and start_noram.inc.
Edited by ccrause