cardem: Report the VCC voltage (if supported)

The SIMtrace2 protocol alwasy contained a field for the VCC voltage,
the cardem firmware just never populated that field, even on those
boards that use the ADC to determine its voltage.

Change-Id: Idcecad553fb36380e916378e1420488acbbfa8e3

firmware/libcommon/source/mode_cardemu.c

@@ -168,6 +168,16 @@ static void card_emu_uart_set_direction(uint8_t uart_chan, bool tx)
 #endif
 }
 
+int card_emu_get_vcc(uint8_t uart_chan)
+{
+	struct cardem_inst *ci = &cardem_inst[uart_chan];
+#ifdef DETECT_VCC_BY_ADC
+	return ci->vcc_uv / 1000;
+#else
+	return -1;
+#endif
+}
+
 /* call-back from card_emu.c to enable/disable transmit and/or receive */
 void card_emu_uart_enable(uint8_t uart_chan, uint8_t rxtx)
 {