Remove extraneous USBCON defines for AT90USB boards

USBCON is definied by serial.h pullikng in MarlinConfig.h which in turn pulls in Arduino.h.  Defining in later includes has shown to cause compile issues so removing all extraneous calls for and potentially easier future linker troubleshooting.
more USBCON unification

Add support for Printrbot Neopixel RGBW strip.

Connected as described at http://printrbot.com/shop/led-strip/

Based on patch by Kelly Anderson <kelly@xilka.com> at
http://www.xilka.com/printrbot/marlin/1.1.4/20170707/

MKS 1.3+: Add pin mapping for PS_ON

Update Russian translation - Part 1

Signed-off-by: Alexey Shvetsov <alexxy@gentoo.org>

Fixing SWITCHING_EXTRUDER feature to work with HOTENDS > 1

This allows the use of D4 as PS_ON since MKS Gen doesn't have a PS_ON pin. However this effectively allows only 3 servos (instead of 4).

Signed-off-by: Alexey Shvetsov <alexxy@gentoo.org>

Improve FWRETRACT comments

Add more details as requested in #7198

Without this...  The LSF won't work because none of the sampled points
in this code block get added.

This was broken by 9af67e2446.

Fixes for recent merges

We can save more and a pile of RAM by eleminating the eqnBVector and
EqnAMatrix arrays next.

Volatile keyword not needed for parser.seen

Stay in User Menu after command. Add feedback.

When a user executes a User Menu command, the LCD returns to the main menu. If the user has multiple menu items they want to run, such as one menu item per bed leveling corner, then it's better to stay in the User Menu rather than return to the main menu.

This PR modifies Marlin to stay in the User Menu after a command.

Updates to G33 and its LCD code

Fix compile error when UBL is enabled but there is no LCD Panel.

Arrange more example configs by vendor

20x4 map integration.    Also some minor changes to the UBL Menu layout.     Both 20x4 LCD's and Graphical LCD panels should have similar functionality now.

Add support for BQ heated bed kit with Hephestos 2

Assign -1 to LCD_PINS_D4-7 if not defined

Support for Creality CR-10

Creality board is a Melzi / Sanguinololu variant, but using different pins for LCD control (and maybe other minor variations)

Use FastIO for spindle, probe test, and case light

"Keepalive" is now "Busy protocol"

Patch M23 to work around Simplify3D bug