kconfig: add CONFIG_CC_COVERAGE

Make it possible to use gcc code coverage analysis.

v1 -> v2:
 - Kconfig: remove not needed 'default n'
 - Makefile: use consistent spacing

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/.gitignore b/.gitignore
index 29757aa..f1b8015 100644
--- a/.gitignore
+++ b/.gitignore
@@ -85,3 +85,7 @@
 *.orig
 *~
 \#*#
+
+# gcc code coverage files
+*.gcda
+*.gcno