runtime: move timeUnit to a single place

The timeUnit is now the same type everywhere. Move it to a single place
and add some documentation to it.

src/runtime/runtime_esp8266.go

@@ -9,8 +9,6 @@ import (
 	"unsafe"
 )
 
-type timeUnit int64
-
 var currentTime timeUnit = 0
 
 func putchar(c byte) {