First cut of configurable serial port functionality.

Currently port usage is hard-coded to the default port layout, cli
commands are coming in a future commit.

This decouples all code from the global 'serialPorts' structure which
has been removed.  Any code that needs to use a serial port can use
findOpenSerialPort() and openSerialPort() and maintain it's own
reference to the port.

Ports can switch between functions.  e.g. by default
cli/msp/telemetry/gps
passthrough all use USART1.  Each port maintains it's current function.
see begin/endSerialPortFunction.

There are only certain combinations of serial port functions that are
supported, these are listed in serialPortFunctionScenario_e.

This commit also adds a few 'static' keywords to variables that should
have been.

There a a few other minor fixes and tweaks to various bits of code that
this uncovered too.
Rename serialrx_type to serialrx_provider to match gps_provider and
telemetry_provider.
Disable some acc/gyros on Olimexino.
Allow the user to specify their serial port scenarios.
Configuration validation.

This enables a new feature setting for PARALLEL_PWM which is enabled by
default.

This starts to move much of the feature checking/excluding code that is
littered through into a single place - validateAndFixConfig().  Since
the config is known to be valid after the method is called other code
can just get on with it's job instead of checking for confliciting
features/settings.
Add callback support to software serial.

Also fixed problem with shared serial port detection.
CHEBUZZF3 - Cleanup USART irq handler.  Disable UART2 DMA for now.