powerpc/boot: Separate CPP flags from BOOTCFLAGS

Add BOOTCPPFLAGS variable for the CPP options required by C and AS.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230606064657.183969-3-npiggin@gmail.com