u-boot.mk: add support for config customization

u-boot.mk: add support for config customization

Make it possible to easily customize U-Boot config options via new
`UBOOT_CUSTOMIZE_CONFIG` variable, so we don't need to patch config
files or override config step with shell hackery.

This generic approach uses `config` CLI to tweak the .config as needed,
for example:

 UBOOT_CUSTOMIZE_CONFIG := \
	--enable CMD_EFIDEBUG \
	--enable CMD_BOOTMENU \
	--enable AUTOBOOT \
	--enable AUTOBOOT_MENU_SHOW \
	--disable AUTOBOOT_KEYED \
	--disable AUTOBOOT_USE_MENUKEY \
	--disable BOOTMENU_DISABLE_UBOOT_CONSOLE \
	--set-val BOOTDELAY 2

Signed-off-by: Petr Štetiar <ynezz@true.cz>