docker: Fix test-mingw

Feature "dtc" is explicitly required by test-mingw, but is not detected
by the run script since we switched to archive-source.sh in b7f404201e4.
Since it isn't available in the Fedora image which runs this test on
patchew, the way we get dtc is still from submodule.

archive-source.sh takes care of bundling the submodule files already, so
what we need to do is just checking if files are there. Makefile is
chosen because it is one that is unlikely to get renamed in the future.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170925082913.22089-1-famz@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
