improvements to writeimage.sh

[AntiSol]

Hi,

Here are some tweaks I made to writeimage.sh:

• decompress images on the fly, no intermediate file (save some disk space / cleanup headaches)
• use pv (if available) to show pretty progress bar for image write (otherwise show dd progress info)
• use dd's direct/sync options, no need to sync all disks (might be marginally faster)
• move superuser check to last (so that other args are parsed and warnings show before running elevated or prompting for password)

Hope you like! :)

[AntiSol]

note that according to the xz man page -T 0 isn't implemented for decompression, but if it were you could addextractopts=(-d -c -T 0) here to support that.