mtd: base-files: Unify dual-firmware devices (Linksys)

mtd: base-files: Unify dual-firmware devices (Linksys)

Consistently handle boot-count reset and upgrade across
ipq40xx, ipq806x, kirkwood, mvebu

Dual-firmware devices often utilize a specific MTD partition
to record the number of times the boot loader has initiated boot.

Most of these devices are NAND, typically with a 2k erase size.
When this code was ported to the ipq40xx platform, the device in hand
used NOR for this partition, with a 16-byte "record" size. As the
implementation of `mtd resetbc` is by-platform, the hard-coded nature
of this change prevented proper operation of a NAND-based device.

* Unified the "NOR" variant with the rest of the Linksys variants

* Added logging to indicate success and failure

* Provided a meaningful return value for scripting

* "Protected" the use of `mtd resetbc` in start-up scripts so that
   failure does not end the boot sequence

* Moved Linksys-specific actions into common `/etc/init.d/bootcount`

For upgrade, these devices need to determine which partition to flash,
as well as set certain U-Boot envirnment variables to change the next
boot to the newly flashed version.

* Moved upgrade-related environment changes out of bootcount

* Combined multiple flashes of environment into single one

* Current-partition detection now handles absence of `boot_part`

Runtime-tested: Linksys EA8300

Signed-off-by: Jeff Kletsky <git-commits@allycomm.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
[checkpatch.pl fixes, traded split strings for 80+ chars per line]