I should have communicated that no use of -DKERNEL is permitted in the boot loader. You're not allowed to compile with that defined at all. You need to condition things on -D_STANDALONE. It's a third environment to compile in. There are big issues defining KERNEL elsewhere in the boot loader (which is why the practice was banned and all other issues corrected). The number of places needing ifdef _STANDALONE is small here.

OK. It too should be under -D_STANDALONE. Although it turns out not to be needed if you don't define KERNEL and make a small tweak to systm.h's confused use of KERNEL.

Yea, this has to be done in a different way. This isn't acceptable. A simple include file would do that trick....

The long list of includes isn't permitted. It's unmaintainable.

Instead, you need to create a facility in defs.mk to allow you to create the proper stack of include files. It's one line there and lets you move these all to #include inside of zstd_shim.c. Most of the defines to 'turn off' include files go away too. I'll add that so there's a regular way to have early args in CFLAGS on a per-target basis.

If you do that, you'll be able to #include these files in zstd_shim.c and have them work as intended.

With these changes, list.c needs no special treatment other than the early include.

nvlist.c runs into the bug in the zfs code where two different boolean_t definitions exist. This should be resolved separately, however. Just adding the freebsd spl directory to CFLAGS is enough to get by, though should be revisited. zfs.c has the same issue. It also doesn't need lz4 to compile.