Correct write_propval_bytes() for platforms with signed char by default

Some platforms (including the Microsoft C compiler) have char defaulting
to signed.  write_propval_bytes() in the -O dts code will not behave
correctly in this case, due to sign extension.

Signed-off-by: Andrei Errapart <andrei@errapartengineering.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

treesource.c

@@ -178,7 +178,7 @@ static void write_propval_bytes(FILE *f, struct data val)
 			m = m->next;
 		}
 
-		fprintf(f, "%02hhx", *bp++);
+		fprintf(f, "%02hhx", (unsigned char)(*bp++));
 		if ((const void *)bp >= propend)
 			break;
 		fprintf(f, " ");