ipq806x: increase kernel partition size for the Netgear Nighthawk X4 R7500v2

ipq806x: increase kernel partition size for the Netgear Nighthawk X4 R7500v2

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 R7500v2.

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
(cherry picked from commit c3af761e4740820a29e993414d3d2f49c7eff6e7)