kernel: drop unnecessary True options from calls to getVar

kernel: drop unnecessary True options from calls to getVar

The older style calls (plus a bashism in kernel.bbclass, fixed
separately) were introduced via the recent change to add support for
multiple kernel packages:

  http://git.openembedded.org/openembedded-core/commit/?id=6c8c899849d101fd1b86aad0b8eed05c7c785924

(From OE-Core rev: e660ef68de3b3891a26ed6e10d96dc4efaf03ffc)

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>