MFC r326315, r326330, r326331, r326412:

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@326534 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f