Fix #216

CMSIS-Atmel 4.5 changed the prescaler defines to match the bit mask, not the actual prescaler value, thus `SYSCTRL_OSC8M_PRESC_1_Val`  now means "divide by 2"

Fixes https://github.com/arduino/ArduinoCore-samd/issues/214

    * cores/arduino/wiring.c:

Use the information retrieved from the linker script in use to simply call a reset without any erase

The exported variable is used by banzai() to avoid erasing any bootloader part

Fixes some compile errors and warnings

This pachs allows C++ global constructors to run after hardware
initialization. This helps some libraries that setups hardware
in class constructor to work properly.

See also #169

Fix #172

Fixed issue where pins were not being configured properly in analogWrite when multiple channels of the same TCC/TC module were being used to drive outputs.

Port of https://github.com/arduino/Arduino/pull/5362

It’s no longer needed with the timeout mechanism in USBCore.

Timeout is checked on the next send attempt. Initial timeout is set to
70ms, after a time out occurs and the previous transfer is still not
complete a send will report as failed. If a send times out, a ZLP will
be in the send queue to check for future timeouts.

This bypass the generic (but inefficient) implementation of Stream::readBytes
and uses an optimized version of readBytes than can do efficient multi-byte reads.

This change improves read performance when massive data is sent
via USB by exploiting the hardware capability to handle multi-packet
transfers autonomously.

It makes no sense to have ::recv calling repeatedly ::read in this
case.

Now the release() function only performs the action that is called for,
i.e. release the endpoint and let it receive data. All the buffers handling
has been inlined in the callers, this also slightly improves performance
because it allows to remove some redundant checks.

The reference to the upper USBDevice class is passed on
the EPHandler constructor.

this is the proper type to encapsulate a byte when data is available
or -1 when there is no data.