git: Add .gitignore for build directory

BUG=none
TEST=make
     git status
     # Should not see build/* artifacts

Change-Id: Id5d2e38e48254ce56327f8e0577a4bc170fe1b45

.gitignore

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..567609b
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+build/