add .gitignore file

Add .gitignore to specify list of temporary files to be ignored by git.

Change-Id: Icdd23874dc22ba69c67bb5cef611d51ccf3b665e
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..6da54be
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+build*/
+subprojects/*
+!subprojects/*.wrap