Add 'license' command to U-Boot command line

The 'license' command includes the U-Boot license (GPLv2) into the
actual bootloader binary. The license text can be shown interactively
at the U-Boot commandline.

For products where the commandline can actually be accessed by the
end user, this helps to prevent inadvertent GPL violations, since the
GPLv2 license text can no longer be 'forgotten' to be included into
the product.

The 'license' command can be enabled by CONFIG_CMD_LICENSE.

Signed-off-by: Harald Welte <laforge@openmoko.org>