generic: routerboot sysfs platform driver

generic: routerboot sysfs platform driver

This driver exposes the data encoded in the "hard_config" flash segment
of MikroTik RouterBOARDs devices. It presents the data in a sysfs folder
named "hard_config". The WLAN calibration data is available on demand via
the 'wlan_data' sysfs file in that folder.

This driver permanently allocates a chunk of RAM as large as the
"hard_config" MTD partition (typically 4KB), although it is technically
possible to operate entirely from the MTD device without using a local
buffer (except when requesting WLAN calibration data), at the cost of a
performance penalty.

This driver does not reuse any of the existing code previously found in
routerboot.c.

This driver has been successfully tested on BE (ath79) and LE (ipq40xx
and ramips) hardware.

Tested-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
Tested-by: Baptiste Jonglez <git@bitsofnetworks.org>
Tested-by: Tobias Schramm <t.schramm@manjaro.org>
Tested-by: Christopher Hill <ch6574@gmail.com>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>