gfxstream: add generate-gfxstream-vulkan.sh

Simpler version of the generate-vulkan-sources.sh
script.

Reviewed-by: Aaron Ruby <aruby@blackberry.com>
Acked-by: Yonggang Luo <luoyonggang@gmail.com>
Acked-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27246>

src/gfxstream/codegen/scripts/cerealgenerator.py

@@ -546,7 +546,7 @@ class BumpPool;
                 "device", "generic", "goldfish-opengl",
                 "system", "vulkan_enc")
         self.guest_abs_encoder_destination = \
-            envGetOrDefault("VK_CEREAL_GUEST_ENCODER_DIR",
+            envGetOrDefault("GFXSTREAM_GUEST_ENCODER_DIR",
                             default_guest_abs_encoder_destination)
 
         default_host_abs_decoder_destination = \
@@ -555,9 +555,9 @@ class BumpPool;
                 "android", "android-emugl", "host",
                 "libs", "libOpenglRender", "vulkan")
         self.host_abs_decoder_destination = \
-            envGetOrDefault("VK_CEREAL_HOST_DECODER_DIR",
+            envGetOrDefault("GFXSTREAM_HOST_DECODER_DIR",
                             default_host_abs_decoder_destination)
-        self.host_script_destination = envGetOrDefault("VK_CEREAL_HOST_SCRIPTS_DIR")
+        self.host_script_destination = envGetOrDefault("GFXSTREAM_SCRIPTS_DIR")
         assert(self.host_script_destination is not None)
 
         self.addGuestEncoderModule(