scons: rework the EGL build

The scons equivalent of the previous commit - just fold the almost
identical driver + main Sconscripts.

Cc: Alexander von Gluck IV <kallisti5@unixzen.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Matt Turner <mattst88@gmail.com>
4 files changed