Currently, there's no way to generate a dpm.img file from the Bazel
build. As this should be generated when a dpm overlay dtbo exists,
generate it in that case.
To make use of this macro, just set dpm_overlay = True when defining
an LA target.
Change-Id: I10222499737ffb9b43cb2bac235ada66c8cae2e5
Signed-off-by: John Moon <quic_johmoo@quicinc.com>