Refactor ramdisk element to allow alternate implementations

In preparation for enabling Dracut-based ramdisks, this change
factors out functionality that is common to both busybox and Dracut
ramdisks.  Said functionality is moved to a ramdisk-base element
which is added as a dependency of the ramdisk element.  ramdisk now
only contains the functionality specific to building busybox-based
ramdisks.

bp tripleo-juno-dracut-ramdisks
Change-Id: Iad2907c8be491c88727d87ed5e5a720e5beb66c3
Merge "Refactor ramdisk element to allow alternate implementations"