uboot-config.bbclass: Don't bail out early in multi configs

Previously we had the support to build multiple u-boot configs for a
machine, but after the change in the commit 801a27d7
("uboot-config.bbclass: Raise error for bad key"), this anonymous
function would bail out after handling the first config in UBOOT_CONFIG.
This is definitely not what we want. Fix it by making sure all the
configs are handled.

Fixed: 801a27d7 ("uboot-config.bbclass: Raise error for bad key")
Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>