scripts: west_commands: Add powershell autocompletion (west -b)

Windows powershell can by default not autocomplete known boards when
tapping which can cause frustration when you can't quite remember the
exact name or you don't want to type it out.
west build .\path\to\application\ -p -b stm<tab> will search for
available board including 'stm' in their name.

Signed-off-by: Tais Hjortshøj <tbh@mjolner.dk>