[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

filesystem/filesystem.go

@@ -35,6 +35,7 @@ func registerBuildComponents(ctx android.RegistrationContext) {
 	ctx.RegisterModuleType("android_filesystem", filesystemFactory)
 	ctx.RegisterModuleType("android_system_image", systemImageFactory)
 	ctx.RegisterModuleType("avb_add_hash_footer", avbAddHashFooterFactory)
+	ctx.RegisterModuleType("avb_gen_vbmeta_image", avbGenVbmetaImageFactory)
 }
 
 type filesystem struct {