linux-firwmare: restore WHENCE_CHKSUM variable

linux-firwmare: restore WHENCE_CHKSUM variable

Restore WHENCE_CHKSUM variable which is used to hold the WHENCE file
checksum. It is necessary to allow easily overriding it from local.conf
if the devupstream version is selected:

PREFERRED_VERSION_linux-firmware = "1:20220708+git%"
SRCREV:class-devupstream = "${AUTOREV}"
WHENCE_CHKSUM:class-devupstream:pn-linux-firmware = "abf1077491eeb261ecdcb680a34fc059"

Without the WHENCE_CHECKSUM one would need to manually patch the
LIC_FILES_CHKSUM variable to change the checksum of WHENC (e.g. using
the anonymous python function or remove expression).

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>