boot/uboot: needs host-{flex,bison}

Recent U-Boot no longer ship the flex/bison generated kconfig parser, as
of commit e91610da7c8a9fe42f3e5a75f06c3d1a0cb5f815 (kconfig: re-sync
with Linux 4.17-rc4).

So, add the conditional kconfig dependencies, as we just did for the
kernel.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/uboot/uboot.mk

@@ -449,5 +449,8 @@ 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_KCONFIG_DEPENDENCIES = \
+	$(BR2_BISON_HOST_DEPENDENCY) \
+	$(BR2_FLEX_HOST_DEPENDENCY)
 $(eval $(kconfig-package))
 endif # BR2_TARGET_UBOOT_BUILD_SYSTEM_LEGACY