Enables signing for generic boot images

Enables signing for them so we can verify that the boot partition
contains a valid generic boot image during the compliance test.

This change also adds the chained partition descriptor of
boot partition into vbmeta.img, which contains the corresponding
public key used to verify the boot partition.

Bug: 151094943
Test: lunch aosp_arm64-user, `make` and use `avbtool info_image`
      to check that GKI images are properly signed.
Test: `make dist` and checks that GKI images in the image zip file
      are properly signed.
Change-Id: I2fd69964f9dc62526a813875fa9193913aa4935b