uart_ow: Initial support for Dallas 1-wire over UART support for MRAA

This commit introduces support for Dallas Semiconductor (DS) 1-wire
compliant device support using an available UART device.

The principle of operation is described in the following Application
note by Maxim Electronics:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/214

with help (1-wire search) from:
https://www.maximintegrated.com/en/app-notes/index.mvp/id/187

It has been tested on Galileo 2 and Edison, with 2 DS 1-wire devices,
the DS18B20 and DS2413 connected to the bus.  A UPM driver for the
DS2413 is already complete and a PR will be submitted after this one.

It is important that you use a UART with CMOS/TTL level voltages
(3.3v/5v) RX and TX lines.  DO NOT use standard RS232 level voltages
or you are going to have a bad day.

In order for this to work, a simple interface circuit, using a single
diode must be constructed:

(forgive my "Asciihematic" :)

-|
U|
A| TX---|<--+
R|          |
T| RX-------o--------o 1-wire data bus
-|

The diode on TX is a 1N4148 (cheap and common), with the cathode
connected to TX, and the anode connected to RX and the 1-wire data
line.

The 1-wire data line requires a pull-up resistor, as the DS 1-wire
spec requires. 4.7-5K is typical for DS 1-wire buses.

NOTE: DHT-type (temp/humidity sensor) 1-wire devices ARE NOT DS 1-wire
compliant, and will not work with this code/circuit unfortunately.

Also note, this will use up one of your UARTs, which cannot be used
for any other purpose (ie: to access true UART-type serial devices).

You can however, connect as many DS 1-wire devices as feasible to this
UART, as it will function as a DS 1-wire bus master.

Signed-off-by: Jon Trulson <jtrulson@ics.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

examples/CMakeLists.txt

@@ -9,6 +9,7 @@ add_executable (spi_mcp4261 spi_mcp4261.c)
 add_executable (mmap-io2 mmap-io2.c)
 add_executable (blink_onboard blink_onboard.c)
 add_executable (uart uart.c)
+add_executable (uart_ow uart_ow.c)
 add_executable (mraa-gpio mraa-gpio.c)
 add_executable (mraa-i2c mraa-i2c.c)
 add_executable (spi_max7219 spi_max7219.c)
@@ -30,6 +31,7 @@ target_link_libraries (spi_mcp4261 mraa)
 target_link_libraries (mmap-io2 mraa)
 target_link_libraries (blink_onboard mraa)
 target_link_libraries (uart mraa)
+target_link_libraries (uart_ow mraa)
 target_link_libraries (mraa-gpio mraa)
 target_link_libraries (mraa-i2c mraa)
 target_link_libraries (spi_max7219 mraa)