Update T GKI certification scheme

* Remove 'generic_ramdisk' certification, as we are not certifying
  'init_boot' anymore.
* Add a new 'boot' certificate that certifies the whole boot.img.
* The new 'boot' & existing 'generic_kernel' certificates are directly
  appended at the end of the boot.img, and the mkbootimg
  --boot_signature argument is removed.

Bug: 211741246
Test: m bootimage
Test: ./boot_signature_info.sh boot-5.10.img
Change-Id: I143680b1cab50a6915df56c8273f8741beaf1180