vim: put xxd in its own package

The xxd tool can be quite handy by itself, and doesn't have anything
to do with vim per se. Make it possible to include the rather tiny xxd
in a rootfs without pulling in the several MB vim binary and
associated data.

For backwards compatibility, add an RDEPENDS from the main package to
the new vim-xxd package.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>