Remove --platform option and export part name to `part_name`.

[Krzysztof Boroński]

Platform option is too ambiguous. Part name should be required to be specified explicitely, then exposed as part_name variable so a correct pinmap could be used.

See

f4pga/f4pga/platforms/xc7a50t.json

Line 101 in 9b140e9

 "map": "${shareDir}/arch/${device}/${part_name}/pinmap.csv", 

It will always default to xc7a35tcsg324-1, unless the user explicitly sets part_name to something else. This makes --platform completely unnecessary.

[Tomasz Michalak]

Fixed with #568