ipq806x: switch to full dual-boot for the nbg6817 sysupgrade support

ipq806x: switch to full dual-boot for the nbg6817 sysupgrade support

Instead of writing to the currently booted partition set, this
implements full dual-boot support for sysupgrade by always writing to
the other, currently inactive, partition set and toggling the dualflag
after a successful flash.

The currently active/ booted partition set is determined by parsing
/proc/cmdline for its rootfs parameter (supplied by the DTS), instead
of reading from the 0:DUAL_FLAG mtd, in order to prevent (potentially)
bricking both partition sets.

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>