MMU: Add workaround for broken Tune button

[gudnimg]

If the MMU is processing a command, the Tune button can't be executed and will put the printer into an unrecoverable state.

Mitigate the issue by ignoring the Tune button if the MMU is processing a command. In 99.9% of cases this will only happen when printing. The Tune button is still useful for the particular use case scenario it was added into the firmware in the first place, which is to allow the user to tune the idler stallguard threshold immediately after the printer boots up.

Fixes #4558

For anyone interested, a proper fix may look like this: #4561 but for now it is better to at least mitigate the issue instead of leaving it open in the firmware.

Steps to reproduce a problem:

1. Send toolchange command to the printer e.g. T0
2. Hold the Idler in place until you get a "Idler cannot home" error.
3. Select 'Tune' button
4. Select 'Done' button (you need to click it 3 times to exit the menu)
5. Printer is in a 'frozen' state.

Change in memory:
Flash: +16 bytes
SRAM: 0 bytes

[github-actions]

All values in bytes. Δ Delta to base

Target	ΔFlash	ΔSRAM	Used Flash	Used SRAM	Free Flash	Free SRAM
MK3S_MULTILANG	16	0	247774	5653	6178	2539
MK3_MULTILANG	16	0	247072	5662	6880	2530

[gudnimg]

@3d-gussner adding to 3.14 milestone just so the issue is tied to some milestone, please free to change it if its not appropriate.

[DRracer]

good enough as a workaround 👍

[3d-gussner]

@gudnimg We can make a cherry pick to MK3_3.14.0 branch, and let do a short test run for it.