disk-installer: add support for installation to disk.

Installer will copy GRUB and SmartOS platform from the CD-ROM, create
SMI labels, modify GRUB's menu and install bootloader in order to make
the pool bootable. Unfortunately, this limits supported disk capacity
to 2TB.

Please note that this code isn't hooked into interactive installer.

Change-Id: I468ec636fb10ff0115c1adce00d5094b8c3db642
Signed-off-by: Piotr Sikora <piotr.sikora@frickle.com>
disk-installer: add option to choose the source for SmartOS files.

"disk_install" config parameter can take one of the following values:
- cd  - copy SmartOS files from the CD,
- usb - copy SmartOS files from the USB key,
- fs  - copy SmartOS files from the hard disk, this option supports
        scenario in which SmartOS installer is booted from the disk
        that will be later wiped in order to create ZFS pool on it.

Change-Id: Ib3d8f67fd8af5622528e104a4ca51952a4770f51
Signed-off-by: Piotr Sikora <piotr.sikora@frickle.com>