Switch the airoha target to use kernel 6.18 and drop 6.12.
This removes a large amount of patches that have since been
merged to upstream; and removes the build system hack used
to support building 6.12 and 6.18 with a slightly different
DTS and package configuration.
Signed-off-by: Kenneth Kasilag <kenneth@kasilag.me>
Link: https://github.com/openwrt/openwrt/pull/23640
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
bc864d2978