build with external bcm library

[jscrane]

this library packages an old bcm library, it would be good to use the pi's own one.

they are in fact both written by Mike McAuley, http://www.airspayce.com/mikem/bcm2835/

the bundled one is v1.8 while the latest is v1.55.

(note that the spi_begin() implementations are very different between the two)

[jscrane]

there doesn't appear to be an official raspian package for this library.

[jscrane]

Have committed branch which compiles against bcm2835-1.59 but doesn't work:

$ sudo ./sensorhub -v
================ SPI Configuration ================
CSN Pin  	 = Custom GPIO22
CE Pin  	 = Custom GPIO7
Clock Speed	 = 8 Mhz
================ NRF Configuration ================
STATUS		 = 0x00 RX_DR=0 TX_DS=0 MAX_RT=0 RX_P_NO=0 TX_FULL=0
RX_ADDR_P0-1	 = 0x0000000000 0x0000000000
RX_ADDR_P2-5	 = 0x00 0x00 0x00 0x00
TX_ADDR		 = 0x0000000000
RX_PW_P0-6	 = 0x00 0x00 0x00 0x00 0x00 0x00
EN_AA		 = 0x00
EN_RXADDR	 = 0x00
RF_CH		 = 0x00
RF_SETUP	 = 0x00
CONFIG		 = 0x00
DYNPD/FEATURE	 = 0x00 0x00
Data Rate	 = 1MBPS
Model		 = nRF24L01
CRC Length	 = Disabled
PA Power	 = PA_MIN