RFC: allow first boot config from msdos partition

Hello,

I've had the idea that it'd be nice to allow more first boot
configuration of our ARM images from the FAT partition.  The machine
I normally use is a MacOSX box, so when writing images, it isn't easy
to put an rc.conf on the image.

This change imports configinit from cperciva's ec2-scripts (one minor
tweak to make it better), and creates an fs_configinit script that
reads the config.init file from /boot/msdos and passes it through
configinit.

This will allow first boot config, like setting a static IP, installing
packages at first boot making it easier to get a system functional.
It also means that if you have a simple appliance, if you package your
config as a config.init script, it's easier to upgrade and test FreeBSD
snapshots and releases than it was previous.

I've written the man pages.  If there is anything that isn't clear or
missing, please let me know.

The changes are in here: https://reviews.freebsd.org/D26713

Thanks!

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."