Implement GCC code coverage analysis

In order to provide some insight on what code is executed during
coreboot's run time and how well our test scenarios work, this
adds code coverage support to coreboot's ram stage. This should
be easily adaptable for payloads, and maybe even romstage.

See http://gcc.gnu.org/onlinedocs/gcc/Gcov.html for
more information.

To instrument coreboot, select CONFIG_COVERAGE ("Code coverage
support") in Kconfig, and recompile coreboot. coreboot will then
store its code coverage information into CBMEM, if possible.
Then, run "cbmem -CV" as root on the target system running the
instrumented coreboot binary. This will create a whole bunch of
.gcda files that contain coverage information. Tar them up, copy
them to your build system machine, and untar them. Then you can
use your favorite coverage utility (gcov, lcov, ...) to visualize
code coverage.

For a sneak peak of what will expect you, please take a look
at http://www.coreboot.org/~stepan/coreboot-coverage/

Change-Id: Ib287d8309878a1f5c4be770c38b1bc0bb3aa6ec7
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/2052
Tested-by: build bot (Jenkins)
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Reviewed-by: Martin Roth <martin@se-eng.com>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/Kconfig

@@ -194,6 +194,15 @@ config REQUIRES_BLOB
 	  coreboot build for such a board can override this manually, but
 	  this option serves as warning that it might fail.
 
+config COVERAGE
+	bool "Code coverage support"
+	depends on COMPILER_GCC
+	default n
+	help
+	  Add code coverage support for coreboot. This will store code
+	  coverage information in CBMEM for extraction from user space.
+	  If unsure, say N.
+
 endmenu
 
 source src/mainboard/Kconfig
@@ -868,6 +877,15 @@ config TRACE
 	  the 0xaaaabbbb is the actual function and 0xccccdddd is EIP
 	  of calling function. Please note some printk releated functions
 	  are omitted from trace to have good looking console dumps.
+
+config DEBUG_COVERAGE
+	bool "Debug code coverage"
+	default n
+	depends on COVERAGE
+	help
+	  If enabled, the code coverage hooks in coreboot will output some
+	  information about the coverage data that is dumped.
+
 endmenu
 
 # These probably belong somewhere else, but they are needed somewhere.