Reinstate Trusty as default for Ubuntu

Now that issues associated with this bump have been addressed
elsewhere, reinstate trusty as the default DIB_RELEASE for
the ubuntu element.  Also adds a note about memory requirements
to the ramdisk element.

Change-Id: Ia89c83ba0ee4ad3bd89b833ba0db08135eac63ed
Merge "Reinstate Trusty as default for Ubuntu"