base.bbclass: Enable using 'make clean' for rebuilds
When something rebuilds say due to ${baselib} changing or some
other key variable, software is often not rebuilt due to the
fact that make detects no dependency change.
By running "make clean" when these changes occur, we can at least try
and ensure the correct rebuilds happen. We use the same checksum check as
autotools to decide if things have changed or not.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
