x86: fix boot error introduced by kbuild

x86 uses target specific assignment of EXTRA_AFLAGS,
EXTRA_CFLAGS - this caused troubles with
introducing asflags-y, ccflags-y.

Fixed the target specific assignments in arch/x86/boot/Makefile
and auditted the rest of the kernel for similar usage.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>