Added software version + git short hash string to firmware.

The Makefile has the version number and generates a version.h which
has VERSION_SW define that looks like "v<VERSION>-g<GIT_SHORT_HASH>".

Software version shows at boot in the serial console.
And shown with command: `version`.

It can also be requested via `information.json`.
```
{
    "ip_address": "192.168.10.247",
    "ip_gateway": "192.168.10.1",
    "ip_netmask": "255.255.255.0",
    "mac_address": "1c:2a:a3:23:00:02",
    "sw_ver": "v0.1.0-gd48235f",
    "hw_ver": "SWGT024-V2.0"
}
```

cmd_parser.c

@@ -16,6 +16,7 @@
 #include "rtl837x_sfr.h"
 #include "rtl837x_stp.h"
 #include "uip/uip.h"
+#include "version.h"
 
 #pragma codeseg BANK1
 #pragma constseg BANK1
@@ -490,6 +491,13 @@ void print_gpio_status(void) {
 	}
 }
 
+// Show software version
+void print_sw_version(void) __banked {
+	print_string("Software version: ");
+	print_string(VERSION_SW);
+	write_char('\n');
+}
+
 
 // Identify command
 void cmd_parser(void) __banked
@@ -695,6 +703,9 @@ void cmd_parser(void) __banked
 					port_eee_status_all();
 			}
 		}
+		if (cmd_compare(0, "version")) {
+			print_sw_version();
+		}
 	}
 }