boot/uboot: allow taking the entire default environment from a text file

By default U-Boot builds a default environment from its own configuration
and a board-specific set of variables. However it also allows to bypass
this entirely and define the entire default environment from a text file.

Expose this feature to Buildroot. This allows to have a file e.g. in
board/.../uboot.env which contains an easy to maintain text file with the
wanted default environment, without patching the U-Boot source code.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

boot/uboot/Config.in

@@ -136,6 +136,35 @@ config BR2_TARGET_UBOOT_CONFIG_FRAGMENT_FILES
 	  that will be merged to the main U-Boot configuration file.
 endif
 
+config BR2_TARGET_UBOOT_DEFAULT_ENV_FILE
+	string "Text file with default environment"
+	select BR2_TARGET_UBOOT_NEEDS_XXD
+	help
+	  Text file containing the variables to be used as the default
+	  environment in U-Boot.
+
+	  If empty, let U-Boot generate the default enviromnent from the
+	  source code and other U-Boot configuration values, which is
+	  the default behaviour.
+
+	  If set to the path to a file, instructs U-Boot to define the
+	  entire U-Boot default environment from the provided file,
+	  disabling all the default behaviour.
+
+	  From the U-Boot documentation:
+
+	    The format is the same as accepted by the mkenvimage tool,
+	    with lines containing key=value pairs. Blank lines and
+	    lines beginning with '#' are ignored.
+
+	  Based on the USE_DEFAULT_ENV_FILE and DEFAULT_ENV_FILE U-Boot
+	  configuration variables.
+
+	  For more info see:
+	  https://docs.u-boot.org/en/latest/usage/environment.html#external-environment-file
+
+	  Requires U-Boot >= v2018.05.
+
 config BR2_TARGET_UBOOT_NEEDS_DTC
 	bool "U-Boot needs dtc"
 	select BR2_PACKAGE_HOST_DTC

boot/uboot/uboot.mk

@@ -390,6 +390,14 @@ UBOOT_KCONFIG_EDITORS = menuconfig xconfig gconfig nconfig
 # override again. In addition, host-ccache is not ready at kconfig
 # time, so use HOSTCC_NOCCACHE.
 UBOOT_KCONFIG_OPTS = $(UBOOT_MAKE_OPTS) HOSTCC="$(HOSTCC_NOCCACHE)" HOSTLDFLAGS=""
+
+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))")
+endef
+endif
 endif # BR2_TARGET_UBOOT_BUILD_SYSTEM_LEGACY
 
 UBOOT_CUSTOM_DTS_PATH = $(call qstrip,$(BR2_TARGET_UBOOT_CUSTOM_DTS_PATH))
@@ -530,6 +538,7 @@ define UBOOT_KCONFIG_FIXUP_CMDS
 	$(UBOOT_ZYNQMP_KCONFIG_PMUFW)
 	$(UBOOT_ZYNQMP_KCONFIG_PM_CFG)
 	$(UBOOT_ZYNQMP_KCONFIG_PSU_INIT)
+	$(UBOOT_KCONFIG_DEFAULT_ENV_FILE)
 endef
 
 ifeq ($(BR2_TARGET_UBOOT)$(BR_BUILDING),yy)