pmb.parse.bootimg: detect mediatek header (MR 1955)

Some Mediatek devices have a special 512-byte header around the zImage
which must be generated so the device boots.

Support for that exists for a while in postmarketOS but detection was
missing. Add that.

test/test_questions.py

@@ -98,6 +98,7 @@ def test_questions_bootimg(args, monkeypatch):
               "pagesize": "2048",
               "cmdline": "bootopt=64S3,32S1,32S1",
               "qcdt": "false",
+              "mtk_mkimage": "false",
               "dtb_second": "false"}
     assert func(args) == output