Makefile: Allow arch post-link hook

This commit allows an architecture to provide a Makefile.postlink whose
u-boot target gets invoked after the u-boot ELF is linked. This will be
of use for MIPS in a following commit.

This mirrors Linux commit fbe6e37dab97 ("kbuild: add arch specific
post-link Makefile").

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: u-boot@lists.denx.de
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Tested-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>