u-boot: Fix handling of CMD_LICENSE in u-boot-mkimage

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.

(From OE-Core rev: f7d4375146af22c72e2bdc9426329cd37f610a47)

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>