kbuild: move asm-offsets.h rule to ./Kbuild ...

Currently, MIPS is the only architecture that needs
include/generated/asm-offsets.h, but we have got ./Kbuild file now.

It is a good reason to move asm-offsets.h rule from arch/mips/Makefile
to ./Kbuild and add dummy asm-offsets.c for the other architectures.
asm-offsets.h would be useful for all the architectures.

This commit does not implement include/generated/bounds.h,
but if necessary, it is easy to implement it.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Add initial EFI architecture support ...

This adds support for running barebox in an EFI environment
on X86 PC hardware.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>