Merged in ordsen/nuttx/feature-pca9540bdp-i2cmultiplexer (pull request #610)

Feature pca9540bdp i2cmultiplexer

* Implement common i2c multiplexer abstraction to be reused by i2c multiplexer devices. Provide common interface with transfer and reset calls to set up virtual i2c busses.

* Remove i2cmux_lower_half.c and headers, will be implemented by each mux driver

* Implement PCA9540BDP I2C multiplexer lower half which uses the common i2c mux abstraction

* Remove application interface and implement common i2c functions

* Remove application interface from pca driver and implement common i2c functions directly in the pca driver

* Use i2cerr

Approved-by: Gregory Nutt <gnutt@nuttx.org>