tools: add xxd (from vim)

U-Boot requires xxd to create the default environment from an external
file as done in uboot-mediatek.
Build xxd (only, not the rest of vim) as part of tools to make sure it
is present on the buildhost.

Reported-by: David Bauer <mail@david-bauer.net>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>