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.
- Drop duplicated `cpu.c` and `cpu_conf.h`: Those are already provided by
`cpu/atmega_common`.
- The higher values for default stack size of `cpu_conf.h` in
`cpu/atmega_common` results in three tests no longer fitting the available RAM
==> Updated the Makefile.ci to skip linking of those tests for the Arduino
Leonardo
Currently the configuration claims that external interrupt INT4 is present on
pin PE7. However, the ATmega32U4 datasheet (section 10.3.4 page 81) contains
the following remark to pin PE7: "Not present on pin-out". This commit removes
the PE7 from the interrupt config.