custom initramfs fix from slim roms

Change-Id: I0d2c889cc17a029741b32f0d80432278c82e6444
Ability to make flashable kernel zips Credits @Derteufel
Fix bootzip. Install signapk to sign the package and some cleanup

Change-Id: Ife5b74e4643ab757e94deba01d384aa2674cddda