Revert "scripts: build_all: stop building mdm targets"

Since the mdm baseline is now fixed and we need to enable
continuous integration, revert
35cb695a
("scripts: build_all: stop building mdm targets").

Change-Id: I161e77670879de2a5d0b9575d2c802620847d46b
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>