linux-postmarketos-rockchip: Add zstd support for ramdisk

Sebastian Fleerrequested to merge
sepp/pmaports:rockchip-kernel-fix into master

It appears that I tested !7269 (merged) with one of my development kernels - linux-postmarketos-rockchip currently doesn't have CONFIG_RD_ZSTD=y.

While at it, move nftables and other netfilter support from module to built-in and add pmb:kconfigcheck-nftables.

fixes !7269 (merged)

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