payloads: Enable building depthcharge as part of the coreboot build

For CHROMEOS builds, depthcharge can be built automatically.
This dependency exists because depthcharge without vboot and subsequent
signing of the image doesn't work very well, and both are keyed to that
flag as well.

Change-Id: Id0195bd3b4e454f382782106d6512469106daac5
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/10924
Tested-by: build bot (Jenkins)

payloads/external/depthcharge/Kconfig.name

@@ -0,0 +1,8 @@
+config PAYLOAD_DEPTHCHARGE
+	bool "Depthcharge"
+	depends on CHROMEOS
+	help
+	  Select this option if you want to build a coreboot image
+	  with a depthcharge payload.
+
+	  See http://coreboot.org/Payloads for more information.