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 <[email protected]>

include/u-boot.mk

@@ -83,6 +83,9 @@ endef
 
 define Build/Configure/U-Boot
 	+$(MAKE) $(PKG_JOBS) -C $(PKG_BUILD_DIR) $(UBOOT_CONFIGURE_VARS) $(UBOOT_CONFIG)_config
+	$(if $(strip $(UBOOT_CUSTOMIZE_CONFIG)),
+		$(PKG_BUILD_DIR)/scripts/config --file $(PKG_BUILD_DIR)/.config $(UBOOT_CUSTOMIZE_CONFIG)
+		+$(MAKE) $(PKG_JOBS) -C $(PKG_BUILD_DIR) $(UBOOT_CONFIGURE_VARS) oldconfig)
 endef
 
 DTC=$(wildcard $(LINUX_DIR)/scripts/dtc/dtc)