system-installer: improve options handling

system-installer: improve options handling

* Move options handling down, to where all the code starts.
* Make it a bit more general, this will make it easier adding more options
  later.
* Add -h option and implement help text.
* Print nice error message in an unsupported option is given, plus print help
  text. The error message will be prefixed with program name.
* Handle "--" separator too.

Change-Id: I5a337b99fce7cb93417341c35368948e872990a9
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>