[libshiftavr] Use volatile addresses and members in callbacks

[pavly-gerges]

The [libuart] and [libadc] use only volatile protocol_callback* var in their callback instances which restricts the interrupt-safety to the actual instance of the callback and not the instance address and its members, so by changing it to volatile protocol_callback *volatile var and declaring the struct protocol_callback members to volatile, the interrupt-safety will include also asynchronous changes to the addresses and member states.

1. The volatile protocol_callback* would add this feature:

volatile protocol_callback* internal_protocol_callback = ...;
protocol_callback finalize_callback = ...;

ISR(_xxx__vector) {
    //Adjusts the callback instance
    *internal_protocol_callback = finalize_callback;
}

2. The protocol_callback *volatile would add this functionality:

protocol_callback *volatile internal_protocol_callback = ...;
protocol_callback *volatile finalize_callback = ...;

ISR(_xxx__vector) {
    //Adjusts the callback instance address to another one
    internal_protocol_callback = finalize_callback;
}

3. The typedef struct { void (*volatile on_command)(vector); } protcol_callback; would add this functionality:

volatile protocol_callback *volatile internal_protocol_callback = ...;
volatile protocol_callback *volatile finalize_callback = ...;

ISR(_xxx__vector) {
    //Adjusts the callback instance address to another one
    internal_protocol_callback->on_command = finalize_callback->on_command;
}