Add support for PULLMODE=NONE to lpf generation

[antoinevg]

At present the compiler only supports the generation of lpf files containing pin attributes of: PULLMODE=UP and PULLMODE=DOWN.

This PR cleans up the code for generating pull modes as well as adding support for generating PULLMODE=NONE from a pinout.json file such as:

{"type": "input",  "name": "button_user", "value": "M14", "pullmode" : "NONE"},

The original implemenation also had a bug whereby pins without a pullmode property would be assigned a pullmode of NONE:

pullmode = (typeof pullmode === 'undefined') ? 'NONE' : pullmode;

This is incorrect as the default pull mode for a lpf file with no pull mode attribute is DOWN.

However this did not cause any problems in practice due to the conditional below it which would only generate a PULLMODE attribute for UP and DOWN:

if (pullmode === 'UP' || pullmode === 'DOWN') {
  code += 'PULLMODE=' + pullmode;
}