- sleep mode must be enabled in firmware with compile flag SLEEP_ENABLE (default: off)
- sleep is activated by UART and left through EXT_PIN_WAKEUP (rising edge)
- sleep is delayed until pending UART TX has completed (typically ACK for SLEEP command)
- added software control for alternative LEDP configuration (RP) through
  PEN (PEN will be low / LEDP off when modem is sleeping)

changed ACI interface to return NACK on faulty CMD (processCmd returns bool now instead of void)