ipq806x: increase kernel partition size for the TP-Link Archer C2600

ipq806x: increase kernel partition size for the TP-Link Archer C2600

The default image does not fit 2 MB anymore, expand os-image partition
to 4 MB.

Upgrading works transparently via sysupgrade in both directions.
Another option would have been to merge "os-image" and "rootfs" into a
single "firmware" partition using MTD_SPLIT_TPLINK_FW, but just
changing the sizes of the existing partitioning has been deemed safer
and actually tested on an affected device; the maximum for rootfs
changes from 27 MB to 25 MB.

Run-tested on TP-Link Archer C2600.

Signed-off-by: Joris de Vries <joris@apptrician.nl>
[slh: extend comments and commit message, rename rootfs]
Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>