Enable local board repository for PIO (#11079)

-Enable boards_dir option in PIO INI
-Remove defunct custom board file operations for PIO auto-build environment
-Remove now-defunct Teensy++ 2.0 PIO entry, replaced by `at90USB1286_DFU` and `at90USB1286_DFU` (using this entry always put AT90USB board boot loaders in jeopardy of being overwritten due to the very small size of the official Teensy++ 2.0 bootloader)
-whitespace

platformio.ini

@@ -20,6 +20,7 @@ src_dir     = Marlin
 build_dir   = .pioenvs
 lib_dir     = .piolib
 libdeps_dir = .piolibdeps
+boards_dir  = buildroot/share/PlatformIO/boards
 env_default = megaatmega2560
 
 [common]
@@ -89,6 +90,7 @@ lib_deps      = ${common.lib_deps}
 lib_ldf_mode  = deep+
 src_filter    = ${common.default_src_filter}
 extra_scripts = pre:buildroot/share/atom/create_custom_upload_command_CDC.py
+monitor_speed = 250000
 
 #
 # AT90USB1286 boards using DFU bootloader
@@ -105,6 +107,7 @@ lib_deps      = ${common.lib_deps}
 lib_ldf_mode  = deep+
 src_filter    = ${common.default_src_filter}
 extra_scripts = pre:buildroot/share/atom/create_custom_upload_command_DFU.py
+monitor_speed = 250000
 
 #
 # Due (Atmel SAM3X8E ARM Cortex-M3)
@@ -271,20 +274,6 @@ lib_ignore    = Adafruit NeoPixel, c1921b4, TMC2130Stepper
 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)
 #