linux/uboot: line-up repository-related configuration options

Although the configuration options for custom repository locations and
versions are very similar between the linux and uboot packages, there are
some minor differences. This patch lines up both packages.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>