[DRIVERS] Serial drivers updated to use the new chardev API.

Serial driver read and write operations now take an unused argument, an
operation offset pointer, to match the new chardev API.
[DRIVERS] M25P80 NOR flash and similar character driver implemented.

The M25P80 NOR flash and similar character driver has been implemented,
following the new character device API, which allows to use the "vmm_chardev_*"
operations for this flash.