Remove 'generic_ramdisk' and add 'boot' certification

* Remove 'generic_ramdisk' certification since we are no longer
  certifying it in VTS.
* Add 'boot' certification which certifies the entire boot.img.
* 'generic_kernel' certification certifies the kernel.img.
* Remove --boot_signature option.
* Change Android T GKI to have fixed 16K boot_signature size, and must
  be appended at the end of the boot image, before appending the AVB

Bug: 211741246
Test: atest --host mkbootimg_test
Test: atest --host retrofit_gki_test
Test: ./ boot-5.10.img
Change-Id: Ibb9f9bb9fba6a82e4c913b8c23ce519816e96eeb
8 files changed