fix distro detection on RHEL

the new distro detection mechanism doesn't match correctly on RHEL.
this leads to SELinux denials because cobblerd tries to create
/tftpboot instead of the correct /var/lib/tftpboot.
Merge pull request #707 from jmaas/release24

Fix for #675. Add signature for SLES10
Merge pull request #705 from aronparsons/release24

critical: fix distro detection on RHEL