mock: added mraa mock platform infra and GPIO implementation

Mock platform allows one to use mraa without having any real HW.

This commit makes necessary foundational changes and implements
GPIO functionality as well as adds respective tests.

Signed-off-by: Alex Tereschenko <alext.mkrs@gmail.com>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

docs/mock.md

@@ -0,0 +1,37 @@
+Mock platform   {#mock}
+=============
+
+Mocking capability allows libmraa user to work with the library without any real
+hardware available. Enabling this requires library recompilation with architecture
+override (see Building section below). When mock is enabled, library simulates
+actual HW operations at the backend so that for the application it looks
+like a usual board. Being implemented at the backend, the functionality is available
+in all language bindings libmraa supports.
+
+Board configuration
+-------------------
+
+This feature is yet in the experimental mode and not all functionality is available.
+Right now we simulate a single generic board with only GPIO (without ISR) working.
+It also reports having an ADC with 10 (std)/12 (max) bit resolution, but the ADC
+functionality itself is not yet implemented.
+
+We plan to develop it further and all [contributions](../CONTRIBUTING.md) are more than welcome.
+
+See the table below for pin layout and features
+
+| MRAA Number | Pin Name |            Notes            |
+|-------------|----------|-----------------------------|
+| 0           | GPIO0    | GPIO pin, no muxing, no ISR |
+
+Building
+--------
+
+Generally all the building steps are the same as listed
+in the [main building guide](./building.md), you just need to set some specific
+CMake options.
+
+### Linux
+
+To build under Linux, follow standard instructions, just make sure to set
+the `-DBUILDARCH="MOCK"` CMake option.