Implement path-based enabling of code coverage.

Native coverage is enabled by setting NATIVE_COVERAGE to true
and specifying a list of paths in the COVERAGE_PATHS
environment variable. Files are exported to a zip file in the
target out directory.

Change-Id: I66a2ddd88e849bec1cc0cdae1b51fe18a007e2c3
diff --git a/core/definitions.mk b/core/definitions.mk
index 2c09910..0c169e4 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -123,6 +123,15 @@
 $(filter true, $(1))
 endef
 
+###########################################################
+## Rule for touching GCNO files.
+###########################################################
+define gcno-touch-rule
+$(2): $(1)
+	touch -c $$@
+endef
+
+###########################################################
 
 ###########################################################
 ## Retrieve the directory of the current makefile