null1: waffle: add 'null' platform

The 'null' platform (WAFFLE_PLATFORM_NULL) works with EGL_PLATFORM_NULL,
KHR_surfaceless_context and EXT_image_dma_buf_import to hide surfaceless
operation behind a waffle window.
In this platform waffle windows contain gbm buffers which GLES renders
into via dmabuf -> EGL image -> GL framebuffer.
The gbm buffers can be displayed via DRM/KMS.

Signed-off-by: Frank Henigman <fjhenigman@google.com>