Make sorting of ramdisk init elements explicit.

We assemble the init script of ramdisks with the fragments from included
elements, alphabetically.
We now place leading numbers on all of the fragments we ship, to make it
more obvious to element authors and downstream users, how the process
works.

Closes-Bug: #1251706

Change-Id: I56b0d42971c8c462eddcfe1769f8124405e1233c