Finer control for using microswitch endstops with IR sensor

[RRP-support]

Give finer control over specifying which axes use a sensor. Something like: M558 P1 X0 Y0 Z1 Where P0 is sensor off and P1/2/XXX is used for probe type (already implemented). X0 Y0 Z1 define which axes use the sensor - 0 for off, 1 for on.

[chrishamm]

Has been implemented in my firmware fork: https://github.com/zombiepantslol/RepRapFirmware/commit/07bc17ef3fd587c385bcde01be7ef49889a5b8e2