Makefile: Add `all` target to MAKE_DTBS

This patch makes some minor changes to `MAKE_DTBS` make macro
and adds `dtbs` target to the `all` make target.

Change-Id: I1c5b4a603ada31d2dac2ed73da9ff707b410dd11
Signed-off-by: Soby Mathew <soby.mathew@arm.com>