Write sdcard img

This is a list of a bunch of small commits resulting in a complete rewrite of write_sdcard_img

This change includes:
  - removing udev rule requirement in favour of passing the device as command line parameter
  - removing the error_out routine in favour of the more traditional && echo && exit 1
  - removing unnecessary calls to sleep, exit, cd, ...
  - removing the unmount calls in favour of a check if the device is mounted somewhere
  - pretty-formatting of error messages and general program output
  - restructuring of the confirmation message to accept ^[yY] as "yes"
  - removing the -k parameter from the gunzip call for the more traditional < > invocation (not everyone has gunzip 1.6 :))
  - setting a bunch of shell options 
    - -e: exit on failed subcommand
    - -u: error on unbound variable
    - -x: if DEBUG is specified in environment, the shell sets tracing mode and ouputs run commands
  - added a bunch of additional checks and tweaks

each of these points is open for discussion, if you don't like any of my changes then tell me and we will figure it out. :) 

In total though, I think this rewrite has improved the script.

Note: I have not included the /bin/sh change from the other pull request in this file, because I was not sure if that would result in some nasty merge conflicts. The change exists in my local versions though, but keep an eye open during merge :)

Thanks, 
Andy