u-boot-update: allow using config fragments

In order to allow other packages and/or derivative distros to configure specific aspects of u-boot-menu it can be useful to allow using config fragments. This patch therefore implements loading config files from /etc/u-boot-menu.d, overriding the values loaded from the default configuration file.

Note: using /etc/default/u-boot.d as the config folder is also possible (grub does that already). Using a separate directory (ie. not under /etc/default) "feels" better to me, but not sure what you would prefer.