fix: external MPM flash

[mha1]

Summary of changes:

• specific pulse timer inits for and pwm toggle mode
• set correct RX/TX polarity for MPM bootloader
• extended flash process timeout from 12.5ms to 100ms as robustness measure

[pfeerick]

@raphaelcoeffic I've only tried this on the TX16S with external MPM, but it has resolved the flashing issue for that. I did a bisect and asked Michael to take a look at it, as it all went horribly wrong at (prior to that, there was the occasional mid-flash failure, but this PR onwards introduced complete inability to flash.

Next up will be TX12, X9D+ and external MPM, and also an STM32 ELRS module.

6aa1fc90c483ad29c8c12e6eba596f276d83aa18 is the first bad commit
commit 6aa1fc90c483ad29c8c12e6eba596f276d83aa18
Author: Raphael Coeffic <raphaelcoeffic@users.noreply.github.com>
Date:   Fri May 5 11:20:29 2023 +0200

    fix: signal polarity for soft-serial & timer based protocols (#3513)

[ParkerEde]

I tested this PR with the Horus X10S Express and the TARANIS QX7S ACCESS. The ext. MPM could be flashed correctly

[mha1]

As a proud owner of a new external RM 4in1 MPM I can confirm flashing the module is not working in main but working with this PR (as it did with my ext MPM flash PC simulator)

[pfeerick]

Perfect timing Maestro!

• RM TX12 w/ iRange 4in1 MPM 🟢
• Frsky X9D+ w/ RM 4in1 MPM 🟢
• RM TX12MK2 w/ HM ES915TX ELRS 🟢
• Jumper T-Pro internal MPM & external RM 4in1 MPM 🟢

[mha1]

Thank you