MFC r326873,r326874,r326875,r326883,r326884,r326885,r326954,r328865

r326873: Script that knows how to put boot blocks onto a device.

r326874: Script to generate minimal boot images for each of the 24 supported

r326875: Add missing of=/dev/${dev}s${s} when installing zfsboot.

r326883: For now, make the gpart commands verbose

r326884: Be a little verbose and list the loader files we're putting on

r326885: Use -h -D in preference to -D so that the serial port gets the
interactive console.

r326954: Support more images (but still no geli)

r328865: Flesh out the creation of sparc64 UFS images.