Generate y2020 pi configs from a template

This removes a bunch of obnoxious redundancy.

Change-Id: I85741b7dfa482a6e95190628eb23d07394a25ede
diff --git a/y2020/config_gen.bzl b/y2020/config_gen.bzl
new file mode 100644
index 0000000..aed1302
--- /dev/null
+++ b/y2020/config_gen.bzl
@@ -0,0 +1,13 @@
+def generate_pi_config(number):
+  native.genrule(
+      name = "generate_pi_config_%d" % (number),
+      srcs = ["y2020_pi_template.json"],
+      outs = ["y2020_pi%d.json" % (number)],
+      cmd = " ".join([
+          "$(location //y2020:generate_pi_config)",
+          "$(location y2020_pi_template.json)",
+          "'{\"NUM\": %d}'" % (number),
+          "$(OUTS)",
+      ]),
+      tools = ["//y2020:generate_pi_config"],
+  )