pmb.parse.bootimg: adjust to new mkbootimg-osm0sis
Adjust to mkbootimg-osm0sis 2021.04.27, where the output files have been renamed: "use correct output names matching mkbootimg args (zImage=Image.gz=kernel, ramdisk.gz=ramdisk)"
Related: https://github.com/osm0sis/mkbootimg/commit/5a01ae54a9fe910ae1ae51b57194a20b6d8ca889
Trivial fix, will merge as soon as CI passes to unblock other MRs.