dragonboards: Allow use of new mesa/main build

Update the db845c/rb5 build to use the mesa/main build targets
if they are available.

We still keep the libGLES_mesa target, so we can still properly
build with AOSP's mesa branch.

This just allows us to easily test upstream mesa.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Change-Id: Icc8e3fedecb72a458a8419a1fe8bbb326fbe4b87
2 files changed