boot/uboot: don't override dependencies

Don't override UBOOT_DEPENDENCIES in a conditional

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 9de2c792e19d572645d1a2e813516fd8e541fa4f)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/uboot/uboot.mk

@@ -500,7 +500,7 @@ UBOOT_DEPENDENCIES += \
 $(eval $(generic-package))
 else ifeq ($(BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG),y)
 UBOOT_MAKE_ENV = $(TARGET_MAKE_ENV)
-UBOOT_KCONFIG_DEPENDENCIES = \
+UBOOT_KCONFIG_DEPENDENCIES += \
 	$(BR2_MAKE_HOST_DEPENDENCY) \
 	$(BR2_BISON_HOST_DEPENDENCY) \
 	$(BR2_FLEX_HOST_DEPENDENCY)