Rename old image file instead of rewrite it

If we have the old image file with same name as new image file
we shouldn't rewrite it, just need to rename old.
Uses date number in old image name.

Fixes TODO in lib/common-functions

Change-Id: Ica58a0f4435217912bddf950322299d2ed2889f3
Correct source-repository comments

git revisions are not references.

Change-Id: If450e70e46cd30a2662b7246a2555c118ed33ad2
Name 99-setup-first-boot uniquely

While an end user would never use both redhat-common and ubuntu
elements at the same time we should always use unique names
for things per DIB conventions.

Change-Id: I2cabc583a1fd41d4fe7fbf8cf93127d49e26aff4
Merge "Name 99-setup-first-boot uniquely"
Merge "Correct source-repository comments"
Merge "Rename old image file instead of rewrite it"