Instead introduce a generic "make prepare" target which is executed if
include/prepare.mk exists.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44329 3c298f89-4303-0410-b956-a3cf2f4a3e73