linux-firmware: add support for building snapshots

linux-firmware: add support for building snapshots

In some cases it is useful to be able to test the snapshot of
linux-firmware (e.g. to test if the updated firmware works on the
particular hardware). Allow building the linux-firmware snapshots.

To switch to the most recent branch, add the following lines to the
local.conf file:

PREFERRED_VERSION_linux-firmware = "1:20220509+git%"
SRCREV:class-devupstream = "${AUTOREV}"
WHENCE_CHKSUM:class-devupstream:pn-linux-firmware = "4288aad55d189fa225d492526b8406d5"

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b023570ae1d239de14b50a0e827582d0e84ddf81)
Signed-off-by: Steve Sakoman <steve@sakoman.com>