boot/uboot: fix custom repo error message

When using a custom git or mercurial repository for u-boot the error message
indicating a version had not been provided incorrectly stated that the URL was
missing. Update the error message to indicate that it's the version that's
missing.

Signed-off-by: Garret Kelly <garret.kelly@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>