The xfa.ld script is incompatible with binutils > 2.35.2 and results
in firmwares that wont boot. Sadly, I couldn't figure out an elegant
way to fix the issue. Instead, I modified the linker script provided
by binutils to also include XFA.
(cherry picked from commit 62d6a560b1b7ac11fcc2ea4c44952a3efb954c30)
cpu.c and startup.c were redundant in most platforms, except for
atmega256rfr2. The common code is now in cpu/atmega_common/cpu.c
and cpu/atmega_common/startup.c. cpu_conf.h is also removed as
it's now in cpu/atmega_common/include thus shared by all atmega
based platforms.
