Restore env:teensy20, for now

See https://github.com/MarlinFirmware/Marlin/pull/11079#issuecomment-399309709

platformio.ini

@@ -275,6 +275,20 @@ src_filter    = ${common.default_src_filter}
 monitor_speed = 250000
 
 #
+# Teensy++ 2.0
+#
+[env:teensy20]
+platform          = teensy
+framework         = arduino
+board             = teensy20pp
+build_flags       = ${common.build_flags}
+#board_build.f_cpu = 20000000L     ; Bug in Arduino framework disallows boards running at 20Mhz
+lib_deps          = ${common.lib_deps}
+lib_ldf_mode      = deep+
+src_filter        = ${common.default_src_filter}
+monitor_speed     = 250000
+
+#
 # Teensy 3.5 / 3.6 (ARM Cortex-M4)
 #
 [env:teensy35]