ipq806x: increase kernel partition size for the Netgear r7800

ipq806x: increase kernel partition size for the Netgear r7800

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 patch is based on a corresponding change by Pavel Kubelun
<be.dissent@gmail.com> and has been tested by Michael Yartys
<michael.yartys@protonmail.com>

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