timeout: factor out wait-for-key-press loop into separate file ...

This patch factors out the wait-for-key-press loop from the shell command
"timeout" into a sparate file, so that it can be used from C, too.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>