ttyrun: add new recipe

ttyrun: add new recipe

ttyrun is a small tool from IBM's s390-tools package to run a command
if the specified TTY is present, and hang if not.

This is useful so that you can list all of the potential terminals in
inittab and not have getty quiting instantly when a device isn't
present, resulting in the "respawning too fast" errors.

Note that DISTRELEASE has to be set as otherwise "build$(DATE)" is used,
which is non-reproducible.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>