powerpc/boot: Separate BOOTCFLAGS from BOOTASFLAGS

BOOTCFLAGS no longer contains anything that BOOTASFLAGS needs (except
-pipe). Separate them to avoid fragility with cross-contamination of
flags which has caused several build problems.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/lkml/CAHk-=whyWUdJDeOBN1hRWYSkQkvzYiQ5RbSW5rJjExgnbSNX9Q@mail.gmail.com/


Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230606064657.183969-4-npiggin@gmail.com