Rename init_boot properties to ro.init_boot.*

Right now they're named ro.bootimage.* which is really confusing.
Change it so that if generic ramdisk is in boot.img, then name the
properties as ro.bootimage.*, otherwise if generic ramdisk is in
init_boot.img, then name the prop as ro.init_boot.*.

Bug: 211823573
Test: m dist
Change-Id: Icff3c7be04b9a3c6490fbf49ba4f05565b029890