ramips: expand storage space on U6 lite

ramips: expand storage space on U6 lite

While still sufficient for the base image, space runs out quickly with
the ~15MiB available in kernel0.  The identical space in kernel1 is
unused, and standard installation instructions flash to both areas so it
isn't an easy fallback to stock either.

Merge the regions for kernel0 and kernel1 so there's more space
available to the user to install packages and some data.

Signed-off-by: Fabian Groffen <grobian@bitzolder.nl>
Link: https://github.com/openwrt/openwrt/pull/22316
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>