wic: Use __file__ instead of sys.argv[0]

wic: Use __file__ instead of sys.argv[0]

Using __file__ makes the code work independently of the way
it's loaded. In some cases wic can be imported by another
program without executing it. sys.argv[0] would not contain
path to the wic in such a cases.

This is an enabler for unit testing with nose framework.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>