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>

include/mraa_internal.h

@@ -62,6 +62,13 @@ mraa_platform_t mraa_x86_platform();
  */
 mraa_platform_t mraa_arm_platform();
 
+/**
+ * setup a mock platform
+ *
+ * @return mraa_platform_t of the init'ed platform
+ */
+mraa_platform_t mraa_mock_platform();
+
 /**
  * runtime detect running usb platform extender
  *