If Marlin is inside the temperature ISR, the stepper ISR is enabled. If
a stepper event is now happening Marlin will proceed with the stepper
ISR. Now, at the end of the stepper ISR, the temperatre ISR gets enabled
again. While Marlin proceed the rest of the temperature ISR, it's now
vulnerable to a second ISR call.

To guarantee that the 5mS pulse from a BLTouch is recognized you need to
have the endstops.update() routine run twice in that 5mS period.

At 200 steps per mm, my system has problems  below a feedrate of 120 mm
per minute.

Two things were done to guarantee the two updates within 5mS:
1) In interrupt mode, a check was added to the temperature ISR.  If the
endstop interrupt flag/counter is active then it'll kick off the endstop
update routine every 1mS until the flag/counter is zero.  This
flag/counter is decremented by the temperature ISR AND by the stepper
ISR.

2) In poling mode, code was added to the stepper ISR that will make sure
the ISR runs about every 1.5mS.  The "extra" ISR runs only check the
endstops.  This was done by grabbing the intended ISR delay and, if it's
over 2.0mS, splitting the intended delay into multiple smaller delays.
The first delay can be up to 2.0mS, the next ones 1.5mS (as needed) and
the last no less than 0.5mS.

=========================================

BLTouch error state recovery

If BLTouch already active when deploying the probe then try to reset it
& clear the probe.

If that doesn't fix it then declare an error.

Also added BLTouch init routine to startup section

to capture chars at UART at 250000 baud.

Also lined up the >> 1 and / 2 mentioned earlier in the unconstrained I term issue,

Also improve the documentation of this and related options, and set
EXTRUDE_MINTEMP to 170 everywhere.

Replace all fastDigitalRead/Write() with READ/WRITE()

Define SCK_PIN, MISO_PIN, MOSI_PIN in pins.h if not already defined in the pins_*.h files.

Remove these conditional pin definitions from the pin-files. They are now always defined.

Define separate sets of SPI-pins for AT90USB with and without Teensy support in fastio.h.
Likely this was the main reason for all that confusion about the AT90USB-SPI pins.