gitignore: add make/ and net/ (with exceptions)

Ignore all files and folders that get created when following the README
and net/README instructions.

Change-Id: I62277f973cc8f1ee61c5244c9d8cc75775c7cfd9
diff --git a/.gitignore b/.gitignore
index c09c878..b6d7fe7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,8 +3,15 @@
 tags
 *.pyc
 make-*
+make/*
 src/*
 !src/fetch-gerrit-patch.sh
 !src/gits
 !src/osmo-add-gerrit-hooks.sh
 !src/README
+net/*
+!net/tmpl_*
+!net/config_2g3g
+!net/common_*
+!net/README
+!net/fill_config.py