Check return code of element-info

Ensure the return code of element-info is actually checked by moving the
export IMAGE_ELEMENT to the subsequent line.

A recent change (Ic0a061995e2ae708c95a535cba6237bff58f7d93) exported the
IMAGE_ELEMENT variable. This caused the return code of element-info to not
actually be checked, since the export command is now checked instead and
almost always will return 0.

This caused a misleading error message when using diskimage-builder with an
unknown element since the script didn't exit when it should have:
$ bin/disk-image-create -a amd64 foobar vm
Building elements: base  foobar vm
ERROR: Element 'foobar' not found in 'bin/../elements'
Expanded element dependencies to:
Building in /var/tmp/image.DuIDY1lW
Please include at least one distribution root element.

Change-Id: I3d9c02bf0b1a4f320280185dd3fab0697d2c92f2
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 "Check return code of element-info"
Merge "Reinstate Trusty as default for Ubuntu"