Build a new init_boot.img that contains the boot ramdisk

This ramdisk used to be in boot.img, and is now placed into this new
init_boot.img instead.
This new image is used for a new init parition to seperate Android
platform artifacts from the kernel artifacts in boot.img.

Test: boot Cuttlefish
Bug: 203698939

Change-Id: Iaaf82486259979ab728730ce72a4e847ae005c18