pmb.parse.bootimg: fix kernel detection
Fix for current testsuite failure. This is critical and trivial, so I'm merging it as soon as the testsuite passes in CI.
Adjust to new text that "file" outputs for ARM linux kernels:
"ARM OpenFirmware FORTH Dictionary..."
Make test_bootimg_kernel pass again, which was failing with:
AssertionError: assert 'heimdall-isorec' in 'File is not an Android ...