initramfs: fix boot with dynamic /dev

Devtmpfs (which is used by devtmpfs/mdev/udev options) doesn't get
automounted by the kernel when an initramfs is used, causing boot
failures when a dynamic /dev is used.

Fix it by adding a pre-init script to mount devtmpfs before running init.

Reported-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>