gem5: use GEM5_RUN_PROFILE in DTB generation

Allows to generate DTBs from profiles other than baremetal.py, which is
the default established by the layer.

Signed-off-by: Adrian Herrera <adrian.herrera@arm.com>
Change-Id: Ic990bd6751e473756d0739f421dce4b8f98248d7
Signed-off-by: Jon Mason <jon.mason@arm.com>