Simplify additional attributes interface

[ccrisan]

Is your improvement proposal related to a problem? Please describe.
Currently, defining additional attributes for a port is very cumbersome:

• special helpers need to be defined to access port's extra info
• saving/restoring from flash is done manually, via memcpy() & friends
• getters also need to update cached value, when reading from flash

Describe the solution you'd like
A simple, concise way to define additional port attributes.