HACK: Makefile: Build a uImage with dtb already appended

Do not commit to mainline; this is a useful hack only for now.

Signed-off-by: R Sricharan <r.sricharan@ti.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>