pmb.parse.bootimg: accept recovery images too in has_mtk_header
Some Mediatek recovery images have headers named "RECOVERY". Accept those too.
Fix suggested by @arzam16
Fixes: #2066 (closed)
Test plan: run bootimg_analyze on image attached to linked issue.
Good normal mediatek boot image for testing (not RECOVERY): mediatek-boot.img
Edited by Alexey Min