boot/uboot: Set MAKE_ENV for kconfig build system

U-Boot fails to build in a GitLab CI context because the kconfig-package
build stage is unable to find bison or flex even though they are
installed in HOST_DIR.

To fix this, set UBOOT_MAKE_ENV so that UBOOT_KCONFIG_MAKE uses the
correct PATH.

Signed-off-by: Thomas Preston <thomas.preston@codethink.co.uk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/uboot/uboot.mk

@@ -449,6 +449,7 @@ endif # BR2_TARGET_UBOOT && BR_BUILDING
 ifeq ($(BR2_TARGET_UBOOT_BUILD_SYSTEM_LEGACY),y)
 $(eval $(generic-package))
 else ifeq ($(BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG),y)
+UBOOT_MAKE_ENV = $(TARGET_MAKE_ENV)
 UBOOT_KCONFIG_DEPENDENCIES = \
 	$(BR2_BISON_HOST_DEPENDENCY) \
 	$(BR2_FLEX_HOST_DEPENDENCY)