new free_contig() and changes to make drivers use it; so now we

have malloc/free, alloc_contig/free_contig and mmap/munmap nicely
paired up.

memory uses malloc/free instead of mmap/munmap as it doesn't have
to be contiguous for the ramdisks (and it might help if it doesn't!).

lib/syslib/alloc_util.c

@@ -73,3 +73,8 @@ void *alloc_contig(size_t len, int flags, phys_bytes *phys)
 	return (void *) buf;
 }
 
+int free_contig(void *addr, size_t len)
+{
+	return munmap(addr, len);
+}
+