include: Add xmalloc, xzmalloc and dma routines

Add a Kconfig value to indicate coreboot builds.

Add prototypes and definitions for:
* dma_coherent
* dma_malloc
* xmalloc
* xzmalloc

Move prototype for memset into stdlib.h from string.h to eliminate build
breaks.

TEST=Build and test on Galileo Gen2

Change-Id: Ib2eb2ca143b0538bdd1863e628af4c1948bc0f8c
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/19207
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/Kconfig

@@ -18,6 +18,10 @@ mainmenu "coreboot configuration"
 
 menu "General setup"
 
+config COREBOOT_BUILD
+	bool
+	default y
+
 config LOCALVERSION
 	string "Local version string"
 	help