Build system: Add cscope target to the Makefile

Fixes arm-software/tf-issues#15

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..8d0fb07
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+cscope.*