Add note suggesting RAMBo users use the RAMBo board type or platformio environment

Cleanup of pins files, Conditionals

Add note that Viki 2.0 wiring diagram v1.2 has a mistake in it;

Suppress warnings

Repair MarlinSerial with TX-buffer

These '{' are important to avoid the redefinition of `unsigned char _sreg = SREG;`
at the same level.
Used in
```
#define CRITICAL_SECTION_START  unsigned char _sreg = SREG; cli();
```

Update french language

MIN_SEGMENTS_FOR_MOVE => MIN_STEPS_PER_SEGMENT

Follow-up to #4666

M211: Enable/Disable Software Endstops

PREVENT_DANGEROUS_EXTRUDE => PREVENT_COLD_EXTRUSION

Also improve the documentation of this and related options, and set
EXTRUDE_MINTEMP to 170 everywhere.

Clean up ultralcd code, dependencies on ULTIPANEL

dropsegments => MIN_SEGMENTS_FOR_MOVE

Optimize, reduce size of MarlinSerial

Fix_Zig_Zag

Improvements to homing / leveling

Add Rambo support/env to platformio.ini; properly maps extended pins

extruder_multiplier => flow_percentage