libva-initial: New bootstrap recipe

Mesa needs libva.pc and libva headers to enable the VAAPI
state tracker and drivers.

This recipe is a variant of the full libva package build as in:
* it only depends on libdrm to build so it doesn't introduce
  the circular dependency between mesa and libva, and
* it doesn't include the libraries in the final package.

However, there is another issue with build dependency handling
in Yocto. libva depends on mesa and mesa depends on this package.
Any package that depends on libva therefore would pull in libva
and this package resulting in an error in the prepare-sysroot
phase because they would install identical files into the
per-recipe sysroot.

Using the package name "*-initial" avoids this because of the
interaction between sstate.bbclass and staging.bbclass: any
package with the pattern "*-initial" in the name is excluded
from the dependency list unless explicitly added to DEPENDS.

Signed-off-by: Böszörményi Zoltán <zboszor@pr.hu>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>