Port of https://github.com/arduino/Arduino/pull/4718 to SAMD core.

This enables WDT, so sketches do not work.

Fix #124

This prevents accidental overwrites of the bootloader if a NULL
pointer is used for writing by mistake.

USBHost used to have an init() method that now is incorporated in
Init(). The leftover init() calls in the library were calling
the init() function in the variant files instead (that has nothing
to do with the USBHost library).

Removing the calls to init() fix it.

Fix #115

See https://github.com/arduino/Arduino/issues/3614