boot/uboot: needs bison/flex even for the legacy buildsystem

When using the legacy buildsystem, the kconfig parser may still be used,
as reported by Thomas:

    >>> uboot 2018.09 Configuring
    ...]
     HOSTCC  scripts/basic/fixdep
     HOSTCC  scripts/kconfig/conf.o
     YACC    scripts/kconfig/zconf.tab.c
    bin/sh: 1: bison: not found
    ake[3]: *** [scripts/kconfig/zconf.tab.c] Error 127
    ake[3]: *** Waiting for unfinished jobs....
     LEX     scripts/kconfig/zconf.lex.c
    bin/sh: 1: flex: not found

However, in that case, the kconfig parser is only generated during the
'configure' step, so we can add bison/flex as standard dependencies.

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

boot/uboot/uboot.mk

@@ -451,6 +451,9 @@ endif # BR2_TARGET_UBOOT_CUSTOM_GIT || BR2_TARGET_UBOOT_CUSTOM_HG
 endif # BR2_TARGET_UBOOT && BR_BUILDING
 
 ifeq ($(BR2_TARGET_UBOOT_BUILD_SYSTEM_LEGACY),y)
+UBOOT_DEPENDENCIES += \
+	$(BR2_BISON_HOST_DEPENDENCY) \
+	$(BR2_FLEX_HOST_DEPENDENCY)
 $(eval $(generic-package))
 else ifeq ($(BR2_TARGET_UBOOT_BUILD_SYSTEM_KCONFIG),y)
 UBOOT_MAKE_ENV = $(TARGET_MAKE_ENV)