powerpc/boot: Fix rebuild when changing kernel endian

Now that we don't set ARCH incorrectly when calling the boot Makefile,
we can use the generic cpp_lds_S rule for converting our zImage.lds.S
into zImage.lds.

The main advantage of using the generic rule is that it correctly uses
if_changed, which means we correctly regenerate the linker script when
switching endian. Fixing that means we are finally able to build one
endian and then rebuild the other endian without requiring to clean
between builds.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>