Adds a standard DONT_CARE pin name that allows specifying that any pin will
work in a pin map.  This is currently used in the NRF51822 serial API to allow
specifying any pins as a UART.
Revert "Adds a standard DONT_CARE pin name that allows specifying that any pin will..."

This reverts commit c804dc8013bb067d0281f082ed28c5e07aca0541.
Disables checking UART pins checking on nRF51822.
Merge pull request #447 from webbbn/nrf51822_uart_any_pin

Adds standard DONT_CARE pin name for specifying that any pin will work