Use common element select-boot-kernel-initrd

Several elements were using almost similar code to
get the current kernel and initrd inside an image.
After factoring out this code into a common element,
modify the elements to make use of it.

Change-Id: Ib547cd96b2b354b1c97342cacf5f0d0a3f70a3da
Merge "Use common element select-boot-kernel-initrd"