For historical reasons (NuttX STM32 numbering) PX4 bus numbering is
1 based. All PX4 code, including, board code is written to assuming 1
based numbering. In the PX4 I2C driver the 1 is subtracted from the
buss number to address the freqency array. These macros are used to
allow the board config to define the bus numbers in terms of the NuttX
driver numbering. 1,2,3 for one based numbering (STM32) or 0,1,2
for zero based (Kinetis)
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