ipq806x: increase kernel partition size for the Netgear Nighthawk X4 R7500

ipq806x: increase kernel partition size for the Netgear Nighthawk X4 R7500

Starting with kernel 4.14 and gcc 7, the kernel doesn't fit into the
2 MB reserved for the kernel partition by the OEM firmware anymore.

This patch increases the kernel partition from 2 MB to 4 MB, at the
expense of the rootfs, for all supported kernels.

WARNING: This breaks sysupgrade! Flashing a firmware containing this
changed partitioning from an older image with 2 MB reserved for the
kernel partion requires the tftp recovery procedure, thereby losing
all settings.

This change follows the functional example of the Netgear r7800, but
has not been runtime tested on a Netgear Nighthawk X4 R7500

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Cc: Jonas Gorski <jonas.gorski@gmail.com>
(cherry picked from commit c228bbe616a52921a9d8fc961b3efcbdd3880c25)