U-Boot allows specifying additional KConfig fragments to be applied on
top of a defconfig. These are usually located in the board
sub-directory.

    make foo_defconfig bar.config baz.config

Add support for specifying additional KConfig fragments using the
UBOOT_CONFIG variable. Treat the first word in UBOOT_CONFIG as the name
of the defconfig, any additional words as additional fragments.

This can be useful to distinguish between different variants of U-Boot
builds such as different RAM, storage (NAND/eMMC), security etc.

While c05c0699 (u-boot.mk: add support for config customization,
2023-06-02) already added means to modify specific KConfig options
directly from the OpenWrt Makefile, leveraging existing fragments the
U-Boot source-dir provides a more convenient way to extend the config
when customising more than just one or two options. Furthermore it is
desirable to leverage existing fragments from upstream U-Boot where they
exist.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>
Link: https://github.com/openwrt/openwrt/pull/18963


Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 49843b31bc601d52c993530ad5ec185d06e623bf)