generic: routerboot sysfs: add support for soft_config

This driver exposes the data encoded in the "soft_config" flash segment
of MikroTik RouterBOARDs devices. It presents the data in a sysfs folder
named "soft_config" through a set of human-and-machine-parseable
attributes. Changes can be discarded by writing 0 to the 'commit'
attribute, or they can be committed to flash storage by writing 1.

This driver does not reuse any of the existing code previously found in
the "rbcfg" utility and makes this utility obsolete by providing a clean
sysfs interface.

Like "rbcfg", this driver requires 4K_SECTORS support since the flash
partition in which these parameters are stored is typically 4KB in size.

Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Tested-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>