skeleton: /etc/fstab: make sure /tmp is world writable and sticky

ramfs (which is used instead of tmpfs if CONFIG_SHMEM isn't enabled in the
kernel configuration), defaults to mode 0755 instead of 01777 like tmpfs
uses.

/tmp should be world writable and sticky, so explictly enforce the mode so
ramfs users gets it correct instead of relying on the defaults.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>