console/vsprintf: Implement snprintf

snprintf is a safe variant of sprintf. To avoid buffer overflows
we shouldn't use sprintf at all. But for now let's start by
implementing snprintf in first place.

Change-Id: Ic17d94b8cd91b72f66b84b0589a06b8abef5e5c9
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/4278
Tested-by: build bot (Jenkins)
Reviewed-by: Peter Stuge <peter@stuge.se>

src/include/string.h

@@ -17,6 +17,7 @@ int memcmp(const void *s1, const void *s2, size_t n);
 void *memchr(const void *s, int c, size_t n);
 #if !defined(__PRE_RAM__)
 int sprintf(char * buf, const char *fmt, ...);
+int snprintf(char * buf, size_t size, const char *fmt, ...);
 #endif
 
 // simple string functions