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
Ramdisks are now built inside a chroot which is built by the normal
image build process. Doing so improves our independence of the
precise state of the build host.
This fixes bug 1194055.
Change-Id: Ibc254fbb9e7b404b5f38c1b35bcde8a4136e8e28
