uart: add function mraa_uart_get_dev_path

Used for getting the path to the character device under linux for uart
device. i.e. "/dev/ttyS0"

Adds paths to existing platforms.
Closes #84

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

src/x86/intel_de3815.c

@@ -140,6 +140,8 @@ mraa_intel_de3815()
     b->spi_bus[0].miso = 12;
     b->spi_bus[0].sclk = 13;
 
+    b->uart_dev_count = 0;
+
     return b;
 error:
     syslog(LOG_CRIT, "de3815: Platform failed to initialise");