buffet: Allow tests to pass in a path to search for command definitions

This allows us to dynamically generate commands to test Buffet with at
runtime.  The read only nature of the file system makes this difficult
with just the statically configured files.

BUG=brillo:172
TEST=unittests, buffet_Registration passes with this change.

Change-Id: I54a92edadbb5c8a5ebad63b8464d87bb21ba5aa2
Reviewed-on: https://chromium-review.googlesource.com/249440
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
diff --git a/buffet/etc/init/buffet.conf b/buffet/etc/init/buffet.conf
index 0747ef5..1e27a53 100644
--- a/buffet/etc/init/buffet.conf
+++ b/buffet/etc/init/buffet.conf
@@ -12,6 +12,7 @@
 env BUFFET_LOG_LEVEL=0
 env BUFFET_STATE_PATH=
 env BUFFET_CONFIG_PATH=
+env BUFFET_TEST_DEFINITIONS_PATH=
 
 pre-start script
   mkdir -m 0755 -p /var/lib/buffet
@@ -24,4 +25,5 @@
 exec minijail0 -i -g buffet -u buffet /usr/bin/buffet \
     --v="${BUFFET_LOG_LEVEL}" \
     --config_path="${BUFFET_CONFIG_PATH}" \
-    --state_path="${BUFFET_STATE_PATH}"
+    --state_path="${BUFFET_STATE_PATH}" \
+    --test_definitions_path="${BUFFET_TEST_DEFINITIONS_PATH}"