checkkconfigsymbols.py: support Kconfig's 'imply' statement

Support the new imply statement in Kconfig.  The imply statement has
been added by commit 237e3ad0 ("Kconfig: Introduce the "imply"
keyword") and is a weak version of a select, but the target symbol can
still be turned off.

Signed-off-by: Andreas Ziegler <andreas.ziegler@fau.de>
Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>