egl: add HYBRIS_EGLPLATFORM along EGL_PLATFORM

Mesa uses EGL_PLATFORM env variables for its own
purposes.
When the EGL_PLATFORM becomes wayland, mesa
will also try to initialize its wayland backend
when instead it should just stick to fbdev.

Signed-off-by: Adrian Negreanu <groleo@gmail.com>
Merge pull request #73 from groleo/for-upstream

egl: add HYBRIS_EGLPLATFORM along EGL_PLATFORM