Externals configuration file

Store configuration of external repositories in a JSON file.

Add command to dump configuration to stdout.

Change-Id: I84d5388d9bd31e4d1c6626260c9418e07446634a
diff --git a/.gitignore b/.gitignore
index a2a4412..5febf4e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 /core_software
+/linux_driver_stack
 /vela