Enable the endstop interrupts feature for LPC176x boards. Although Smoothieboard chose to use non-interrupt capable pins for their endstops, and this has been copied by clones, so they can't use it.

Co-Authored-By: p3p <p3p@p3psoft.co.uk>

Always honor minimum period on stepper pulse generation, and fix timing calculations

Signed-off-by: etagle <ejtagle@hotmail.com>

- 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.

Pointed out by @GMagician

* Misc fixes and improvements

- Get rid of most critical sections on the Serial port drivers for AVR and DUE. Proper usage of FIFOs should allow interrupts to stay enabled without harm to queuing and dequeuing.
  Also, with 8-bit indices (for AVR) and up to 32-bit indices (for ARM), there is no need to protect reads and writes to those indices.
- Simplify the XON/XOFF logic quite a bit. Much cleaner now (both for AVR and ARM)
- Prevent a race condition (edge case) that could happen when estimating the proper value for the stepper timer (by reading it) and writing the calculated value for the time to the next ISR by disabling interrupts in those critical and small sections of the code - The problem could lead to lost steps.
- Fix dual endstops not properly homing bug (maybe).

* Set position immediately when possible

Don't build and link with different flags, the binary may not work.

* Use a different method to find the volume info in Windows

- Also implemented real endstop reading on interrupt.