i2c: mraa_i2c_frequency now returns correct returns

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
uart: add pre uart hook

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>
edison: fix uart missing information

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>