powerpc/boot: Clean up Makefile after cflags and asflags separation

Tidy pass over boot Makefile. Move variables together where possible.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230606064657.183969-5-npiggin@gmail.com