kconfiglib.py: Kludge in 'imply' support

Currently upstream does not yet understand the imply keyword.  For what
we use kconfiglib.py for today, this is OK.  We only need to be able to
evaluate in order to make boards.cfg and none of those choices will
depend on how imply evaluates out.

Signed-off-by: Tom Rini <trini@konsulko.com>