loop_sdcard: Add loopable SD card file sections

To support continuous belt printing, add nestable repeat
loop support via an `[sdcard_loop]` module.

Supported G-Code:
  - SDCARD_LOOP_BEGIN COUNT=n  ; Loop for N times, or infinitely if N is 0
  - SDCARD_LOOP_END            ; End of loop
  - SDCARD_LOOP_DESIST         ; Complete all loops without iterating

Marlin M808 compatibility example in `config/sample-macros.cfg`:
  - M808 Ln        ; Loop for N times, or infinitely if N is 0
  - M808           ; End of loop
  - M808 K         ; Complete all loops without iterating

Added unit tests in test/klippy/sdcard_loop.test

See https://reprap.org/wiki/G-code#M808:_Set_or_Goto_Repeat_Marker

Signed-off-by: Jason S. McMullan <jason.mcmullan@gmail.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/docs/Config_Reference.md b/docs/Config_Reference.md
index 30c2468a..812dd7a9 100644
--- a/docs/Config_Reference.md
+++ b/docs/Config_Reference.md
@@ -1312,6 +1312,20 @@ path:
 #   be provided.
 ```
 
+## [sdcard_loop]
+
+Some printers with stage-clearing features, such as a part ejector or
+a belt printer, can find use in looping sections of the sdcard file.
+(For example, to print the same part over and over, or repeat the
+a section of a part for a chain or other repeated pattern).
+
+See the `config/sample-macros.cfg` file for a Marlin compatible M808
+G-Code macro.
+
+```
+[sdcard_loop]
+```
+
 ## [force_move]
 
 Support manually moving stepper motors for diagnostic purposes. Note,