skeleton: fix absence of /dev/shm on static, read-only systems

/dev/shm is needed for systems using shared memory. On init-based systems
this directory is usually created in the inittab along with /dev/pts, by
the lines:

package/busybox/inittab:
  ::sysinit:/bin/mkdir -p /dev/pts
  ::sysinit:/bin/mkdir -p /dev/shm

package/sysvinit/inittab:
  si2::sysinit:/bin/mkdir -p /dev/pts
  si3::sysinit:/bin/mkdir -p /dev/shm

However this is broken when static /dev management is selected and the root
filesystem is read-only, showing during boot the error:

  mkdir: can't create directory '/dev/shm': Read-only file system

Fix it by creating the empty /dev/shm directory, just like /dev/pts.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>