u-boot: add option to specify config fragments

U-Boot nowadays also uses kconfig, so we can handle config fragments like we
do for barebox/busybox/linux.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

boot/uboot/Config.in

@@ -129,6 +129,12 @@ config BR2_TARGET_UBOOT_CUSTOM_CONFIG_FILE
 	depends on BR2_TARGET_UBOOT_USE_CUSTOM_CONFIG
 	help
 	  Path to the U-Boot configuration file.
+
+config BR2_TARGET_UBOOT_CONFIG_FRAGMENT_FILES
+	string "Additional configuration fragment files"
+	help
+	  A space-separated list of configuration fragment files,
+	  that will be merged to the main U-Boot configuration file.
 endif
 
 config BR2_TARGET_UBOOT_NEEDS_DTC

boot/uboot/uboot.mk

@@ -196,6 +196,7 @@ else ifeq ($(BR2_TARGET_UBOOT_USE_CUSTOM_CONFIG),y)
 UBOOT_KCONFIG_FILE = $(call qstrip,$(BR2_TARGET_UBOOT_CUSTOM_CONFIG_FILE))
 endif # BR2_TARGET_UBOOT_USE_DEFCONFIG
 
+UBOOT_KCONFIG_FRAGMENT_FILES = $(call qstrip,$(BR2_TARGET_UBOOT_CONFIG_FRAGMENT_FILES))
 UBOOT_KCONFIG_EDITORS = menuconfig xconfig gconfig nconfig
 UBOOT_KCONFIG_OPTS = $(UBOOT_MAKE_OPTS)
 define UBOOT_HELP_CMDS