Wait up to 3 seconds for the data volume to initialize

When storage hardware takes too long to initialize, the disk is not
available at the time automount runs so the persistent boot2docker
volume never gets mounted.  By adding a `waitusb` option to the kernel
command line, we can pause the init sequence and allow the hardware
initialization and discovery to complete.

`waitusb=3:LABEL=boot2docker-data` will cause Tiny Core Linux to pause
until a volume with the `boot2docker-data` label is availalbe or 3
seconds have passed.
Merge pull request #401 from sykesm/waitusb-for-data

Wait up to 3 seconds for the data volume to initialize