x86/boot: implement early command line parser in C

x86/boot: implement early command line parser in C

Current early command line parser implementation in assembler
is very difficult to change to relocatable stuff using segment
registers. This requires a lot of changes in very weird and
fragile code. So, reimplement this functionality in C. This
way code will be relocatable out of the box (without playing
with segment registers) and much easier to maintain.

Additionally, put all common cmdline.c and reloc.c definitions
into defs.h header. This way we do not duplicate needlessly
some stuff.

And finally remove unused xen/include/asm-x86/config.h
header from reloc.c dependencies.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>