Add X_BED_SIZE, Y_BED_SIZE to configurations

Add PIO entry for PRINTRBOARD_REVF

Remove extraneous USBCON defines for AT90USB boards

fix from PIO corruption

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 9af67e24.

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.