Add Flash Device API

[aaron.nyholm at unfoldedeffective.com]

Attached is the updated flashdev API. I've focused on the feedback
previously provided. I've added called the missing calls to bring
this API inline with that of the zephyr example provided. The
formatting is brought inline with RTEMS style. The shell command is
extended to include all previous and added API calls. Included is a
testsuite which tests the API calls and wraps a virtual flash device.

I want to thank you all for your understanding and feedback on my
first submission. I'm more than happy to make further changes to 
this submission based on feedback to provide an acceptable addition.

Thanks,
Aaron.