x86_64: use default block2mtd timeout of 5s

If the underlying block device is a USB device it might require some
time to settle, observed when booting a squashfs image from sd card
on an Alix APU board.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42966 3c298f89-4303-0410-b956-a3cf2f4a3e73