cbfs: alternative support for cbfs_load_payload()

In certain situations boot speed can be increased by providing an
alternative implementation to cbfs_load_payload(). The
ALT_CBFS_LOAD_PAYLOAD option allows for the mainboard or chipset to
provide its own implementation.

Booted baskingridge board with alternative and regular
cbfs_load_payload().

Change-Id: I547ac9881a82bacbdb3bbdf38088dfcc22fd0c2c
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/2782
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc.jones@se-eng.com>

src/Kconfig

@@ -47,6 +47,15 @@ config CBFS_PREFIX
 	  Select the prefix to all files put into the image. It's "fallback"
 	  by default, "normal" is a common alternative.
 
+config ALT_CBFS_LOAD_PAYLOAD
+	bool "Use alternative cbfs_load_payload() implementation."
+	default n
+	help
+	 Either board or southbridge provide an alternative cbfs_load_payload()
+	 implementation. This may be used, for example, if accessing the ROM
+	 through memory-mapped I/O is slow and a faster alternative can be
+	 provided.
+
 choice
 	prompt "Compiler to use"
 	default COMPILER_GCC