vulkan: Add build-install_libvulkan virtual branch

Change-Id: I8f028c6aedf6bcc44d1da25cf9b0e16b95f3c10b
(cherry picked from commit d25223b2a824b8b78d6e0783069aecb8981d9428)
diff --git a/vulkan/patches/README b/vulkan/patches/README
index 64ccf5d..d424dd8 100644
--- a/vulkan/patches/README
+++ b/vulkan/patches/README
@@ -12,8 +12,15 @@
 
 frameworks_base-apk_library_dir
 -------------------------------
-This branch is for frameworks/base. It modifies the framework to
+This branch is for $TOP/frameworks/base. It modifies the framework to
 inform the Vulkan loader, during activity startup, where the
 activity's native library directory. The loader will search this
 directory for layer libraries. Without this change, layers will only
 be loaded from a global location under /data.
+
+
+build-install_libvulkan
+-----------------------
+This branch is for $TOP/build. It adds libvulkan.so to the base
+PRODUCT_PACKAGES variable, so it will be built and installed on the system
+partition by default.