Add FreeBSD 64-bit boot support.

Tested with a FreeBSD 10-b1 image and Debian kFreeBSD 7.2.

Note that grub does not read in FreeBSD loader variables
by default. It can be done using the 'kfreebsd_loadenv'
command to source a file, or by setting grub env variables
prefixed by 'kFreeBSD.' The prefix is stripped by the grub
freebsd boot routine, and the env vars are then passed to
the FreeBSD kernel.

To use a serial console for Debian/kFreeBSD, the following
env variables must be set:

set kFreeBSD.hint.uart.0.at=isa
set kFreeBSD.hint.uart.0.port=0x3F8
set kFreeBSD.hint.uart.0.flags=0x10
set kFreeBSD.hint.uart.0.irq=4

.. which are the required lines from a stock FreeBSD /boot/device.hints
file.

In addition, the '-h' parameter needs to be passed to the kfreebsd
command. A sample command line is

kfreebsd -h /boot/kernel/kfreebsd-9.gz

Note: Debian/kFreeBSD's FreeBSD-8 system won't boot - the AHCI driver
in that release doesn't support MSI, and nor is there a virto driver.
The 9.0-based system doesn't have a virtio net driver, but will correctly
install from CD onto an AHCI hard drive. The remaining issue there is
getting a login on the serial port post-install - the kFreeBSD folk
will probably know how to fix that.
Merge branch 'master' of https://github.com/grehan-freebsd/grub2-bhyve