Reduce warnings/errors in libpayload when using picky compiler options

The new build system uses quite a few more -W flags for the compiler by
default than the old one. And that's for the better.

Change-Id: Ia8e3d28fb35c56760c2bd0983046c7067e8c5dd6
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/72
Tested-by: build bot (Jenkins)
Reviewed-by: Uwe Hermann <uwe@hermann-uwe.de>

payloads/libpayload/Config.in

@@ -73,6 +73,10 @@ config TARGET_POWERPC
 
 endchoice
 
+config MEMMAP_RAM_ONLY
+	bool "Only consider RAM entries in memory map for further processing"
+	default n
+
 config MULTIBOOT
 	bool "Multiboot header support"
 	depends on TARGET_I386