emulator: Enable GLES emulation libraries

This patch enables the build of OpenGLES emulation libraries
(both in the guest and the host) for any emulator-specific
product config.

+ update core/main.mk to ensure that the right path is parsed
  when building the Windows SDK

Change-Id: Ifde047f8c0ef922504a7286c8c5b57f7c79c71f8