Allow variant to override the default rise time in nanoseconds via
define

When write is called and TX buffer is full.

Make sercom4 and sercom5 optional to allow compilation of samd21e variants.

USB Configuration was meant to run only once, but if the board comes back from standby the host can reconfigure the device again.
Probably a cleaner patch could be free()-ing the buffers on standby() to release the memory but at least we don't leak anymore.

Fixes https://github.com/arduino/ArduinoCore-samd/issues/293

This commit prepares to enlarge the ringbuffer for specific needs (at compile time).
Also, it fixes a spurious include in delay.h

INPUT_PULDOWN was not suitable to completely discharge the input capacitor on ADC_BATTERY.
Switching to OUTPUT, LOW discharge it faster then try to read

The bootloader side will disable charging on boot by default.