powerpc/boot: Parse chosen/cmdline-timeout parameter

On some platforms a 5 second timeout during boot might be quite long, so
make it configurable. Run the loop at least once to let the user stop
the boot by holding a key pressed. If the timeout is set to 0, don't
wait for input, which can be used as a workaround if the boot hangs on
random data coming in on the serial port.

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
[mpe: Changelog wording & whitespace]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>