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

Barebox starting from 2019.02 no longer ships flex/bison generated
parser. Add conditional kconfig dependencies, same as we did for kernel
and uboot.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

boot/barebox/barebox.mk

@@ -76,6 +76,10 @@ $(1)_KCONFIG_FRAGMENT_FILES = $$(call qstrip,$$(BR2_TARGET_$(1)_CONFIG_FRAGMENT_
 $(1)_KCONFIG_EDITORS = menuconfig xconfig gconfig nconfig
 $(1)_KCONFIG_OPTS = $$($(1)_MAKE_FLAGS)
 
+$(1)_KCONFIG_DEPENDENCIES = \
+	$(BR2_BISON_HOST_DEPENDENCY) \
+	$(BR2_FLEX_HOST_DEPENDENCY)
+
 ifeq ($$(BR2_TARGET_$(1)_BAREBOXENV),y)
 define $(1)_BUILD_BAREBOXENV_CMDS
 	$$(TARGET_CC) $$(TARGET_CFLAGS) $$(TARGET_LDFLAGS) -o $$(@D)/bareboxenv \