bitbake: bin/bitbake: add a check for /dev/shm existing

bitbake: bin/bitbake: add a check for /dev/shm existing

Python multiprocessing needs /dev/shm, and if it doesn't exist (e.g. in
a chroot environment) or is not writable, you'll get
"OSError: [Errno 38] Function not implemented" or
"OSError: [Errno 13] Permission denied" with a traceback, which doesn't
really help to explain what is wrong.

Implements [YOCTO #4407].

(Bitbake rev: e7460d01f281f913a94192e1f4bd20688164bdd4)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>