[avb_footer] Add avb_gen_vbmeta_image to generate vbmeta image

This is the reland of the change aosp/2375848. The build of initrd on
linux-x86 that has incorrect format (b/264940248) is disabled in this
cl.

Bug: 260821553
Test: m microdroid_kernel_signed and inspect the output using `avbtool
    info_image --image <output>`
Change-Id: I3ad2419b7132cde4b2fc34ddfa09ec5ba2166819