ramips: support dual image feature on ALFA Network boards

New U-Boot version for MediaTek MT76x8/MT762x based ALFA Network boards
includes support for a 'dual image' feature. Users can enable it using
U-Boot environment variable 'dual_image' ('1' -> enabled).

When 'dual image' feature is enabled, U-Boot will modify DTB and divide
the original 'firmware' flash area into two, equal in size and aligned
to 64 KB partitions: 'firmware' and 'backup'. U-Boot will also adjust
size of 'firmware' area to match installed flash chip size.

U-Boot will load kernel from active partition which is marked with env
variable 'bootactive' ('1' -> first partition, '2' -> second partition)
and rename both partitions accordingly ('firmware' <-> 'backup').

There are 3 additional env variables used to control 'dual image' mode:
- bootlimit   - maximum number of unsuccessful boot tries (default: '3')
- bootcount   - current number of boot tries
- bootchanged - flag which informs that active partition was changed; if
                it is set and 'bootcount' reaches 'bootlimit' value,
                U-Boot will start web-based recovery which then updates
                both partitions with provided image

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>

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