boot/ports: Provide getter function for flash area object access

The commit provides set of getter functions that allow to
access fields of flash_area and flash_sectors objects.
Usage of these function, instead of direct field access, allows
to keep common code intact when internal, system specific,
implementation changes.
The commit contains the implementation of getters for following
ports: cpress, mbed, mynewt, zephyr.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>