dump_constants: Add a tool to generate a JSON file with all constants

This change adds a way to export all the necessary constants so that
external tools can compile the policy files.

Bug: chromium:856315
Test: make all

Change-Id: I8637c63b9b09839616aa66ef1714444cb4e124ee
diff --git a/.gitignore b/.gitignore
index b500f2a..2414029 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,8 @@
 *.o
 *.a
 
+/constants.json
+
 # Coverage-related files that appear when building with MODE=profiling.
 *.gcno
 *.gcda
@@ -17,6 +19,7 @@
 *.so
 
 # Executables when compiling in-tree.
+/dump_constants
 /libminijail_unittest
 /minijail0
 /minijail0_cli_unittest