Filesystem labeling, paritition sizing, and multiple partition support

I've developed a set of modifications to crochet-freebsd that might be of interest.  They:

1. Allow one to specify sizes and labels for FAT and UFS partitions

2. Allow one to create multiple UFS partitions and run the set of FREEBSD phases on a subset of them.

3. Load geom_label on BeagleBone by default, so labels can be referenced in /etc/fstab

The details of (2) above are in the commit message of 1dbcecd.

There is an example config at https://github.com/pkelsey/crochet-configs/blob/master/BBB-multi-install-config.sh that shows how I've used the new features to create the kind of image a typical product might want to use - a boot partition plus two world-installed UFS partitions (see strategy routines at the bottom).