nanobsd: Fix parallel make

[jlduran]

Summary

The NANO_NCPU (number of CPUs) value gets ignored by the build script
when setting the NANO_PMAKE (parallel make) value.

Fix it by setting the NANO_PMAKE later in the process.

PR: 277269
MFC after: 1 month

[jlduran]

I'd be tempted to remove NANO_PMAKE from embedded/common, rescue/common, and dhcpd/common.

[jlduran]

• Move the check for -DNO_CLEAN (-n), so this value is reflected when exporting the variable