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Fixed RH_RF22 so that lastRssi() returns the signal strength in dBm. Suggested by Steve Childress.
Added support for getLastPreambleTime() to RH_RF69. Requested by Steve Childress.
RH_NRF24::init() now checks if there is a device connected and responding, else init() will fail. Suggested by Steve Brown.
RHSoftwareSPI now initialises default values for SPI pins MOSI = 12, MISO = 11 and SCK = 13.
Fixed some problems that prevented RH_NRF24 working with mixed software and hardware SPI  on different devices: a race condition due to slow SPI transfers and fast acknowledgement.