Blackfin: only check for os log when we have external memory

If the part has no external memory configured, then there will be no os
log for us to check, and any attempt to access that memory will trigger
hardware errors.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

arch/blackfin/lib/board.c

@@ -390,7 +390,7 @@ void board_init_r(gd_t * id, ulong dest_addr)
 		post_run(NULL, POST_RAM | post_bootmode_get(0));
 #endif
 
-	if (bfin_os_log_check()) {
+	if (CONFIG_MEM_SIZE && bfin_os_log_check()) {
 		puts("\nLog buffer from operating system:\n");
 		bfin_os_log_dump();
 		puts("\n");