iso element to build bootable ISO images

This commit adds a new element named 'iso' to build a bootable
ISO image for the kernel/ramdisk emitted by the 'baremetal' or
'ramdisk' element.

Change-Id: I89d175a29e2d0bc64b47fe527f0d0f6875f6849a
Merge "iso element to build bootable ISO images"