Point at apt-sources element in variables.

This helps folk see how to use DIB_COMMON_ELEMENTS to optimise build
speed.

Change-Id: I32f290ff20953514a0a20516108051e71d196cb6
Use DIB_COMMON_ELEMENTS in ramdisk creation.

DIB_COMMON_ELEMENTS is used for things like distribution mirror choice
- not using it for ramdisk creation is sufficient to break ramdisk
creation in some environments.

Change-Id: Ic97d537f0c31343606829fadb5ae97aa026495f2
Merge "Point at apt-sources element in variables."
Merge "Use DIB_COMMON_ELEMENTS in ramdisk creation."