- Enable / disable interrupts with `__iSeiRetVal` / `__iCliRetVal`
- Add STM32F1 to Travis CI tests

An undocumented hw bug makes the UART lose chars when RX ISR is disabled, even for a very small amount of time. This happens when RX_BUFFER > 256, and the result is corrupted received commands. Solved by implementing pseudo-atomic operations on 16bit indices.

[2.0.x] Adaptive multiaxis step smoothing, and tons of fixes

The timing value should be properly set for ALL boards. The compiler will check and set maximum step rate calculations based on those values.

- Stepper bugs fixed
- Support MIXING_EXTRUDER with Linear Advance
- Miscellaneous cleanup

[2.0.x] Serial handshaking fixes / debug

Pointed out by @GMagician

Followup to #10920, addressing #10962

As noted in #10965

130048 is available on genuine Teensy2.0++ with Paul's "Halfkay" bootloader.  Atmel DFU-equiped boards have only 122880 available

Co-Authored-By: akunt