Renamed NOSOURCE variable to SOURCE.

I don't like anti-options :)

This should be a transparent change because of
setup.d/99_provide-source::4, which handles both the old and new
variable names:

    -if [ 'true' = "$NOSOURCE" ] ; then
    +if [ 'false' = "$SOURCE" ] || [ 'true' = '$NOSOURCE' ] ; then

This way, we do the correct thing even if the user is using an
outdated version of freedom-maker that hasn't renamed NOSOURCE yet.
This can be cleaned up later, but after we're sure people have
transitioned to an updated FM version.  By my count, they should have
till the end of the year.  Realistically, everybody who cares should
be upgraded within a few weeks to a month at most.
Added dated TODO for removing old variable.
Merge with pere's upstream.
Tee rpi-update output.

This way, we'll be able to see whether rpi-update was successful
without opening the generated image itself.
Correct use of tee command.
Merge pull request #28 from NickDaly/master

Tee rpi-update output.  Rename NOSOURCE variable to SOURCE.