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
diff --git a/target/board/generic/BoardConfig.mk b/target/board/generic/BoardConfig.mk
index 3ba648d..c8a0569 100644
--- a/target/board/generic/BoardConfig.mk
+++ b/target/board/generic/BoardConfig.mk
@@ -36,3 +36,6 @@
 ifeq ($(HOST_OS),linux)
 WITH_DEXPREOPT := true
 endif
+
+# Build OpenGLES emulation guest and host libraries
+BUILD_EMULATOR_OPENGL := true