powerpc/boot: pass CONFIG options in a simpler and more robust way

Commit 5e9dcb61 ("powerpc/boot: Expose Kconfig symbols to wrapper")
was wrong, but commit e41b93a6 ("powerpc/boot: Fix build failures
with -j 1") was also wrong.

The correct dependency is:

  $(obj)/serial.o: $(obj)/autoconf.h

However, I do not see the reason why we need to copy autoconf.h to
arch/power/boot/. Nor do I see consistency in the way of passing
CONFIG options.

decompress.c references CONFIG_KERNEL_GZIP and CONFIG_KERNEL_XZ, which
are passed via the command line.

serial.c includes autoconf.h to reference a couple of CONFIG options,
but this is fragile because we often forget to include "autoconf.h"
from source files.

In fact, it is already broken.

ppc_asm.h references CONFIG_PPC_8xx, but utils.S is not given any way
to access CONFIG options. So, CONFIG_PPC_8xx is never defined here.

Pass $(LINUXINCLUDE) to make sure CONFIG options are accessible from
all .c and .S files in arch/powerpc/boot/.

I also removed the -traditional flag to make include/linux/kconfig.h
work. This flag makes the preprocessor imitate the behavior of the
pre-standard C compiler, but I do not understand why it is necessary.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190705100144.28785-2-yamada.masahiro@socionext.com