boot: Add logic to enable booting from fallback option

The "fallback" extlinux config option allows us to set an alternative
default boot option for when it has been detected that the default is
failing. Implement the logic required to boot from this option when
desired.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

cmd/sysboot.c

@@ -105,7 +105,7 @@ static int do_sysboot(struct cmd_tbl *cmdtp, int flag, int argc,
 	}
 
 	if (pxe_setup_ctx(&ctx, cmdtp, sysboot_read_file, &info, true,
-			  filename, false)) {
+			  filename, false, false)) {
 		printf("Out of memory\n");
 		return CMD_RET_FAILURE;
 	}