boot/uboot: fix Kconfig warning for DEFAULT_ENV_FILE

The following warning occurs from commit "boot/uboot: allow taking the
entire default environment from a text file".

  boot/uboot/Config.in:141:warning: config symbol
  'BR2_TARGET_UBOOT_DEFAULT_ENV_FILE' uses select, but is not bool or
  tristate

Add a hidden 'bool' config so we can enable dependencies.

In addition, the DEFAULT_ENV_FILE works by setting Kconfig options,
therefore we also need to depend on
BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG.

Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/uboot/Config.in

@@ -136,9 +136,16 @@ config BR2_TARGET_UBOOT_CONFIG_FRAGMENT_FILES
 	  that will be merged to the main U-Boot configuration file.
 endif
 
+# Hidden boolean that allows to select BR2_TARGET_UBOOT_NEEDS_XXD when
+# needed.
+config BR2_TARGET_UBOOT_DEFAULT_ENV_FILE_ENABLED
+	bool
+	default y if BR2_TARGET_UBOOT_DEFAULT_ENV_FILE != ""
+	select BR2_TARGET_UBOOT_NEEDS_XXD
+
 config BR2_TARGET_UBOOT_DEFAULT_ENV_FILE
 	string "Text file with default environment"
-	select BR2_TARGET_UBOOT_NEEDS_XXD
+	depends on BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG
 	help
 	  Text file containing the variables to be used as the default
 	  environment in U-Boot.

boot/uboot/uboot.mk

@@ -391,8 +391,8 @@ UBOOT_KCONFIG_EDITORS = menuconfig xconfig gconfig nconfig
 # time, so use HOSTCC_NOCCACHE.
 UBOOT_KCONFIG_OPTS = $(UBOOT_MAKE_OPTS) HOSTCC="$(HOSTCC_NOCCACHE)" HOSTLDFLAGS=""
 
+ifeq ($(BR2_TARGET_UBOOT_DEFAULT_ENV_FILE_ENABLED),y)
 UBOOT_DEFAULT_ENV_FILE = $(call qstrip,$(BR2_TARGET_UBOOT_DEFAULT_ENV_FILE))
-ifneq ($(UBOOT_DEFAULT_ENV_FILE),)
 define UBOOT_KCONFIG_DEFAULT_ENV_FILE
 	$(call KCONFIG_SET_OPT,CONFIG_USE_DEFAULT_ENV_FILE,y)
 	$(call KCONFIG_SET_OPT,CONFIG_DEFAULT_ENV_FILE,"$(shell readlink -f $(UBOOT_DEFAULT_ENV_FILE))")