Use enum __packed in favour of -fshort-enums

Use enum __packed in favour of -fshort-enums

Some architectures like ARM encode the short enum option state in the
object file and the linker checks that this option is consistent for all
objects of an executable.  In case applications use -fno-short-enums,
then this leads to linker warnings.  Use the enum __packed attribute for
the relevent enums to avoid the -fshort-enums compiler option.  This
attribute is at least available on GCC, LLVM/clang and the Intel
compiler.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>