u-boot: unbreaks u-boot for all other layers

The u-boot bbappend broke u-boot builds for other layers due to
replacing the SRC_URI without any overrides.

Fix this by using a namespaced recipe for the at91 u-boot.

This should fix https://github.com/linux4sam/meta-atmel/issues/8

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>