add enum baudrates (#376)

* add enum baudrates

fixes perceived limitation of available baud rates https://github.com/Microsoft/pxt/issues/1752

* correct block comment

* comma correction and remove trailing space

That will teach me just to do cut and paste via github web interface :(

* without running pxt buildtarget

libs/core/serial.cpp

@@ -15,8 +15,26 @@ enum SerialPin {
 enum BaudRate {
   //% block=115200
   BaudRate115200 = 115200,
+  //% block=57600
+  BaudRate57600 = 57600,
+  //% block=38400
+  BaudRate38400 = 38400,
+  //% block=28800
+  BaudRate28800 = 28800,
+  //% block=19200
+  BaudRate19200 = 19200,
+  //% block=14400
+  BaudRate14400 = 14400,
   //% block=9600
-  BaudRate9600 = 9600
+  BaudRate9600 = 9600,
+  //% block=4800
+  BaudRate4800 = 4800,
+  //% block=2400
+  BaudRate2400 = 2400,
+  //% block=1200
+  BaudRate1200 = 1200,
+  //% block=300
+  BaudRate300 = 300
 };
 
 enum Delimiters {