cpu/arm/k60: Add --start-group/--end-group to linker command.

Helps avoiding having to reorder the filenames on the command line when
linking.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: Add upload Makefile target.

make upload is shorthand for make projectname.u

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Add __attribute__((unused)) to default fault handlers.

Was causing compiler warnings on LLVM/Clang and the compiler should
probably be made aware that we need those handlers even though they are
never referenced from actual code.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>