make reserved-memory work on mips, and improve test

tests/tftpboot/script.expect

@@ -14,9 +14,17 @@ while {[gets $fh line] >= 0} {
 close $fh
 
 expect {
-   "s6-linux-init" { exit 0 }
+   "s6-linux-init" { }
    "Reserved memory: unsupported node format, ignoring" { exit 1 }
    timeout { exit 1 }
 }
 
+expect {
+   "#" { send "test -f /proc/device-tree/reserved-memory/phram-rootfs*/name && tty \r" }
+   timeout { exit 1 }
+}
 
+expect {
+   "/dev/ttyS0" { exit 0 }
+   timeout { exit 1 }
+}