release.7: Update EMDEDDEDPORTS documentation

EMDEDDEDPORTS does not contain devel/subversion anymore.

Fixes: a03128832c6c In extra_chroot_setup(), use 'uname -U' to determine OSVERSION.
MFC after: 1 week

(cherry picked from commit 6542fd4b959fa2919b39690ea3ea83e55848e091)