Fix permissions for /etc and other dirs after clean flash (#6039)

During the first OpenWrt boot after a clean flash (when the jffs2
partition is not yet initialized) tmpfs is used instead of the usual
jffs2 overlay filesystem.  If this tmpfs is mounted with default
options, all directories created there (/etc, /etc/config, other
subdirectories in /etc) get permissions 1777 by default, and these
permissions then persist in the created jffs2 at least until
subsequent sysupgrade.  Mounting tmpfs with mode=0755 fixes the
permission problem.

Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>

SVN-Revision: 23906