delay accessing misc partition until its device exists

When the misc partition is on an emmc device, recovery can get started
and try to access it before the kernel has actually created the
device.  Try statting the device before reading or writing it; delay
up to 10 seconds waiting for the device to exist.

Change-Id: Ib9bf6c35fa2c28fc43aa7550aaaffb76c9f6e120