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.

I have repeatedly had this issue across multiple boot2docker releases when running under VMware.  Looking through the issues list, it seems others have had similar issues:

boot2docker/boot2docker#390
boot2docker/boot2docker#26
boot2docker/boot2docker#51

While many of those were closed, I'm not sure all were resolved.

To help address this issue, I propose we add `waitusb=3:LABEL=boot2docker-data` to the kernel command line.  That option causes  Tiny Core Linux to pause until a volume with the `boot2docker-data` label is availalbe or 3 seconds have passed.  If a disk is available with the expected volume label, we shouldn't see a elongated boot time; when a volume with the expected label does not exist (boot without a persistent file system, for example), we will see a 3 second delay in the boot sequence.  This seems like a good tradeoff to me.