image_signing: set_channel: use new lsbval helper
image_signing: set_channel: use new lsbval helper

Minor clean up to the logic.

BUG=None
TEST=`./set_channel recovery_image.bin stable-channel` changed the lsb-release file to stable
BRANCH=None

Change-Id: Idf12b643f88e373b528b50e269537b861052b448
Reviewed-on: https://chromium-review.googlesource.com/414225
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>

coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot