bootstd: extend dtb search paths

Some distros install their dtbs in /dtbs, add an additional search path
for this.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
diff --git a/boot/bootmeth_efi.c b/boot/bootmeth_efi.c
index ae936c8..94c3a18 100644
--- a/boot/bootmeth_efi.c
+++ b/boot/bootmeth_efi.c
@@ -206,6 +206,9 @@
 	case 2:
 		prefix = "/dtb/current";
 		break;
+	case 3:
+		prefix = "/dtbs";
+		break;
 	default:
 		return log_msg_ret("pref", -EINVAL);
 	}