ar71xx: switch CPE/WBS 210/510 to okli-loader

ar71xx: switch CPE/WBS 210/510 to okli-loader

We recently increased the kernel partition size of the CPE/WBS 210/510.
This works fine for new installations of the factory image, but on
sysupgrades, the partition table read by the bootloader is not adjusted.
This limits the maximum size of the kernel loaded by the bootloader to the
old partition size.

While adjusting the partition table would be a cleanest solution, such a
migration would have to happen before an upgrade to a new version with a
newer kernel. This is error-prone and would require a two-step upgrade, as
we mark the partition table partition read-only.

Instead, switch from the lzma-loader with embedded kernel to the
okli-loader, so only the tiny lzma-loader is loaded by the bootloader as
"kernel", and the lzma-loader will then load the rest of the kernel by
itself.

Fixes: e39847ea2f70 ("ar71xx: increase kernel partition size for CPE/WBS 210/510")
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>