build: Generate ext3 images if it is define in BoardConfig.mk

Generate both yaffs2 and ext3 file system images if defined in
BoardConfig.mk instead of one or another.  This allows both system
and userdata images to be generated that will work for nand boot
and emmc boot scenarios.

Change-Id: If4f37c702e71979f744d095d241f3ca4371e894a