u-boot: Fix handling of CMD_LICENSE in u-boot-mkimage
Only ever change the CMD_LICENSE in the u-boot config if it is enabled
to prevent corruption of the config file. In case the CMD_LICENSE was
already disabled, the sed substitution inserted another "# " in front
of the CMD_LICENSE which triggered a restart of Kconfig oldconfig and
failed the build.
Signed-off-by: Marek Vasut <marex@denx.de> Signed-off-by: Ross Burton <ross.burton@intel.com>
