admin: Write out correct version fields in boot/loader/entries files

Before, we were writing the "bootversion", which is either 0 or 1, for
all entries.  This is completely wrong; the idea of the "version"
field is to compare between entries.

Fix this by writing out the inverted index - internally, index 0 is
the *first* boot entry, so we give it the highest version number, and
index N is the last, so give it version 0.

Then fix the deployment sorting code to correctly reverse the version
number comparison, so we read back the right order.

In practice before this bug didn't matter because "normally" you only
have at most two deployments.

https://bugzilla.gnome.org/show_bug.cgi?id=706546