MFC r326315, r326330, r326331, r326412:

r326315:
Set DISTDIR and WRKDIRPREFIX when building ports within the
chroot(8) to avoid mtime changes within the ports checkout,
which can cause checksum differences.

r326330:
Add a comment to release/release.conf.sample documenting
EMBEDDEDPORTS. [1]

Remove and update stale documentation from release(7) while here.

r326331:
Correct a comment.

r326412:
Fix port build flags passed to make(1) after r326315, where
it was missed for embedded image builds.

PR: 206344 [1]
Sponsored by: The FreeBSD Foundation

Update the SRCBRANCH for release.sh to use stable/11 as the
SRCBRANCH.

This is a direct commit to stable/11.

Approved by: re (kib)
Sponsored by: The FreeBSD Foundation