boot/uboot: add support for u-boot-initial-env install

This file is needed by libubootenv for fw_printenv/fw_setenv and
library API's to function when the environment is empty.

See details:
https://github.com/sbabic/libubootenv/tree/v0.3.4#replacement-old-tools

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Reviewed-by: Andreas Naumann <anaumann@ultratronik.de>
Tested-by: Andreas Naumann <anaumann@ultratronik.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/uboot/Config.in

@@ -506,6 +506,13 @@ config BR2_TARGET_UBOOT_SPL_NAME
 	  and TPL name is tpl/u-boot-tpl.bin but not always. SPL name is
 	  MLO on OMAP and SPL on i.MX6 for example.
 
+config BR2_TARGET_UBOOT_INITIAL_ENV
+	bool "Install u-boot-initial-env"
+	help
+	  Install the U-Boot u-boot-initial-env on the target.
+
+	  This file is typically required by libubootenv.
+
 config BR2_TARGET_UBOOT_ZYNQMP
 	bool "Boot on the Xilinx ZynqMP SoCs"
 	depends on BR2_aarch64

boot/uboot/uboot.mk

@@ -138,6 +138,14 @@ UBOOT_MAKE_TARGET += u-boot.stm32
 endif
 endif
 
+ifeq ($(BR2_TARGET_UBOOT_INITIAL_ENV),y)
+UBOOT_MAKE_TARGET += u-boot-initial-env
+define UBOOT_INSTALL_UBOOT_INITIAL_ENV
+	$(INSTALL) -D -m 0644 $(@D)/u-boot-initial-env $(TARGET_DIR)/etc/u-boot-initial-env
+endef
+UBOOT_POST_INSTALL_TARGET_HOOKS += UBOOT_INSTALL_UBOOT_INITIAL_ENV
+endif
+
 ifeq ($(BR2_TARGET_UBOOT_FORMAT_CUSTOM),y)
 UBOOT_BINS += $(call qstrip,$(BR2_TARGET_UBOOT_FORMAT_CUSTOM_NAME))
 endif