sdl: Fix host variant dependencies

The host variant of sdl inherited the dependencies and conf opt from
sdl which could lead to some trouble.

For example:
  If you were to build host-sdl and had alsa-lib enabled in your
  packages list, it would try to build host-alsa-lib which does not
  exists.

This patch define basic dependencies and conf opt for the host variant.

Reported-by: Eric Limpens <limpens@gmail.com>
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>