machine: add Device constant

This field contains the microcontroller name that we're compiling for,
or "generic" if we're not running on a microcontroller.

src/machine/machine_generic.go

@@ -4,6 +4,8 @@ package machine
 
 // Dummy machine package that calls out to external functions.
 
+const deviceName = "generic"
+
 var (
 	UART0 = &UART{0}
 	USB   = &UART{100}