Fix Serial port path regression on Linux, and add docs

Author: gfwilliams

ChangeLog

@@ -111,6 +111,7 @@
             Ensure `Date.getTimezoneOffset()` returns the correct timezone offset (fix #1515)
             Search for and execute files '.boot0'/1/2/3 in Storage at boot time if they exist
             Pixl.js: reduce saved code area to 9 x 4kb to allow for extra features
+            Fix Serial port path regression on Linux, and add docs
 
      1v99 : Increase jslMatch error buffer size to handle "UNFINISHED TEMPLATE LITERAL" string (#1426)
             nRF5x: Make FlashWrite cope with flash writes > 4k

src/jsserial.c

@@ -104,6 +104,9 @@ bool jsserialPopulateUSARTInfo(
       {"cts", JSV_PIN, &inf->pinCTS},
       {"bytesize", JSV_INTEGER, &inf->bytesize},
       {"stopbits", JSV_INTEGER, &inf->stopbits},
+#ifdef LINUX
+      {"path", JSV_STRING_0, 0}, // not used - just here to avoid errors
+#endif
       {"parity", JSV_OBJECT /* a variable */, &parity},
       {"flow", JSV_OBJECT /* a variable */, &flow},
       {"errors", JSV_BOOLEAN, &inf->errorHandling},
@@ -147,7 +150,6 @@ bool jsserialPopulateUSARTInfo(
   }
   jsvUnLock(parity);
   jsvUnLock(flow);
-
   return ok;
 }
 

src/jswrap_serial.c

@@ -248,6 +248,9 @@ storage in the queue. If you're intending to receive a lot of malformed
 data then the queue might overflow `E.getErrorFlags()` would return `FIFO_FULL`.
 However if you need to respond to `framing` or `parity` errors then 
 you'll need to use `errors:true` when initialising serial.
+
+On Linux builds there is no default Serial device, so you must specify
+a path to a device - for instance: `Serial1.setup(9600,{path:"/dev/ttyACM0"})`
 */
 void jswrap_serial_setup(JsVar *parent, JsVar *baud, JsVar *options) {
   IOEventFlags device = jsiGetDeviceFromClass(parent);
@@ -259,7 +262,6 @@ void jswrap_serial_setup(JsVar *parent, JsVar *baud, JsVar *options) {
     jsvLockAgain(options);
 
   bool ok = jsserialPopulateUSARTInfo(&inf, baud, options);
-
 #ifdef LINUX
   if (ok && jsvIsObject(options))
     jsvObjectSetChildAndUnLock(parent, "path", jsvObjectGetChild(options, "path", 0));