Fix: current_command_args contains command

Protect MarlinSerial against interrupts
by shielding the CRITICAL_SECTIONs

Now with a test if RX_BUFFER_SIZE is in the required range.
Code in peek() and read() is now optimized for readability and showing the similarity between the two.

Maybe a bit overprotected in checkRx() and store_char(), but now some days without detected errors from this source.

@thinkyhead
Hi Scott,
your code from a0f362c@thinkyhead is great, but you forgot to skip the
command.
Symptom `M117 Test`shows `M117 Test` on LCD instead of `Test`.

see also
https://github.com/MarlinFirmware/Marlin/commit/a0f362c735
6f8e3c2f17 lines 941, 2851 and so on

Greetings and welcome back

Jochen

Remove obsolete STEPS_MM_E setting

* Configuration.h:
  * updated header comment
  * new UUID feature used
  * sync to default config
  * typos fixed
  * changes better documented
  * reflect change in SDSUPPORT feature
* Configuration_adv.h
  * updated header comment
  * thermal protection tuned to work with K8200
  * homing tuned
  * sync to default config
  * typos fixed
  * changes better documented
  * reflect change in LCD features
  * use long filenames
  * use Watchdog
  * use Babystepping
  * use Auto Filament Change

This formalizes the dependency of MANUAL_BED_LEVELING on
MESH_BED_LEVELING

more suitable display format for delta printer

Repair N in ADVANCED_OK

Rc bug fix

Replace the "ok" in M280, M301, M304, M851 with output not including …

Drop the experimental Configurator

Sanity-check deprecated options replaced by SPI_SPEED

Use next open E connector for Y2/Z2

Remove references to marlinfirmware.org

- Include aligned comment about new SDSUPPORT behavior

Fix handling of spaces in GCode

Development moved to https://github.com/thinkyhead/MarlinConfigurator

The domain is gone and doesn’t redirect, so for now show no URL on the boot screen.

Rename fpos_t to avoid libc conflict

Fix AUTOTEMP documentation in configs

This is a proposed change. This section of M114 output is a debug
extension according to RepRap wiki.

Addresses issue #2715

Send the correct line number.
If no N was in the command - non't send an N.

Replace the "ok" in M280, M301, M304, M851 with output not including an "ok"
to avoid senting two "ok"s for a single g-code.

Doing the same for M105 would break the input filters of the hosts.
For M105 the other "ok" is omitted.

Rc bug fix

Rc bug fix

Instead of changing SDEXTRASLOW variable to a slower speed,  the new
SDULTRASLOW is used for rigidbot panel.