ipq806x: increase kernel partition size for the Netgear Nighthawk X4 D7800

ipq806x: increase kernel partition size for the Netgear Nighthawk X4 D7800

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 D7800.

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Cc: Tathagata Das <tathagata@alumnux.com>
(cherry picked from commit 45b8a7c1a6415125f4fe872ec83b3d31a2fa799b)