This increases kernel partition size and fixes rootfs (file-system)
partition size on TP-Link RE450 v1. Also, while we are at it, switch
from statically defined kernel and rootfs partitions in kernel cmdline
to "tplink-fw" mtd splitter.
Fixes: FS#1072. Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>