cros-constants: add ARC++ variables.

Add ARC_PREFIX and ARC_VENDOR_DIR variables to the cros-constants
eclass so ebuilds that install ARC++ components can use them.

BUG=b:67717927
TEST=emerge arc-mesa

Change-Id: Ice791e6621d27afe145b2490dc7eabbf255ffd6b
Reviewed-on: https://chromium-review.googlesource.com/829979
Commit-Ready: Nicolas Norvez <norvez@chromium.org>
Tested-by: Nicolas Norvez <norvez@chromium.org>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Nicolas Norvez <norvez@chromium.org>
diff --git a/eclass/cros-constants.eclass b/eclass/cros-constants.eclass
index 4e89ad9..9251ddb 100644
--- a/eclass/cros-constants.eclass
+++ b/eclass/cros-constants.eclass
@@ -50,3 +50,14 @@
 # @DESCRIPTION:
 # Path to location of chromite executable directory in the chroot.
 CHROMITE_BIN_DIR="${CHROMITE_DIR}/bin"
+
+# @ECLASS-VARIABLE: ARC_PREFIX
+# @DESCRIPTION:
+# Path to root directory of ARC++ install (relative to sysroot).
+ARC_PREFIX="/opt/google/containers/android"
+
+# @ECLASS-VARIABLE: ARC_VENDOR_DIR
+# @DESCRIPTION:
+# Path to install directory for temporary /vendor build files (relative to
+# sysroot).
+ARC_VENDOR_DIR="/build/rootfs${ARC_PREFIX}/vendor"