Add console kernel parameters to extlinux configuration

Those parameters allow to access the console for VMs using
extlinux (CentOS 6 for example). They are already added to
Fedora images.

Change-Id: I035170bd101062e004fb3782b5f8e5a7944a4f03
Closes-Bug: 1296899
Adds "element-provides" support to element dependencies

An element can define a list of elements provided by itself. This allows
using an alternative element and keeping element dependencies at the
same time. This will be needed for example for mariadb element which will be
an alternative to mysql element - mysql dependency can be kept everywhere
but user can choose mariadb explicitly as a replacement by putting "mysql"
into "element-provides" file.

Change-Id: I7f193612f279dd71d9f1f1edfc60009838a00c5a
Remove an excess cp of disk images.

save_image is used to copy kernel and ramdisks out of the image, which
we will sometimes want to keep the source, and sometimes not. However
for the main image itself, the temp copy is never kept, so use mv
  rather than cp and avoid the excess IO.

Change-Id: I5a9f0d69ffee3e6b872a8927537ac17f02f5aa4d
Merge "Remove an excess cp of disk images."
Merge "Adds "element-provides" support to element dependencies"
Merge "Add console kernel parameters to extlinux configuration"
Revert "Only create a tmpfs big enough for DIB_MIN_TMPFS"

This didn't pass CI and is now causing failures as it moves us back to a
4G tmpfs.

Change-Id: Ie92fd909ad21216b1fda55c21457893df38e882f