Avoid depending on system headers from the source tree during bootstrap

[arichardson]

Summary

This can cause surprising errors if the build tools is built against
headers that don't match the host system. It is also required in order
to allow building on non-FreeBSD systems where the headers in
/usr/include/sys are usually completely incompatible with those in the
source tree.

I added an error to Makefile.boot if this is done and found this was
only the case in libnv. With this error in the Makefile ABI breakages
such as r336019 should no longer be possible.

[arichardson]

Update condition to also handle WITH_CDDL builds which include sys/cddl/compat headers during bootstrap

[arichardson]

Remove debug message

[bdrewery]

Yup. I suggest building with BOOTSTRAPPING=0 to ensure this still passes when all tools get built.

[bdrewery]

@imp may want to see this too

[imp]

lib/libnv/Makefile

12

BOOTSTRAPPING isn't used this way elsewhere in the tree...

[arichardson]

• Fixed condition to also handle BOOTSTRAPPING=0 (libmd includes sys/crypto/sha2)
• Removed the ifndef BOOTSTRAPPING since the sys/nv.h header will always be present in WORLDTMP/usr/include/sys

[kevans]

This will need to be rebased (libnv part committed in rS337549 and MFC'd already due to actual breakage in stable/11), but this looks good otherwise.