Makefile: move fs/common.mk above external.mk

The file external.mk was included before fs/common.mk, so it was impossible
to add rootfs targets using the BR2_EXTERNAL mechanism.
This change moves the inclusion of fs/common.mk before external.mk to allow
this.

Signed-off-by: Philippe Reynes <tremyfr@yahoo.fr>
[ThomasDS: rebased, update commit message]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>