Makefile: provide separate install targets

Currently `make install` will install the binaries, libraries and
includes.

This change separates the install target into install-bin, install-lib
and install-includes, so we have more flexibility, particularly when
we're just using libfdt.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>