In fact, these variable are set as environment variables since we run
"make TARGET=xxx TARGET_ARCH=xxx"; TARGET and TARGET_ARCH are declared
in the command line arguments. However, if you are not make(1) guy,
it is hard to understand that TARGET/TARGET_ARCH are appropriately set
in a chroot environment (as environment variables).
Now, the only environment variable need to be set explicitly is 'PATH'.
If we set PATH in /mk script, we can make pristine sandbox for release
build (i.e., "env -i /usr/sbin/chroot ${CHROOT} /mk" will work).
Valuable comments about this issue from: ru
Tested on (virtually): snapshots.jp.FreeBSD.org
MFC after: 2 weeks
04b9282ff8