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>