Extend and document the "wait" command

The wait command was currently implemented but
undocumented and only used internally. This change
adds a second option toi (optionally) override the default
timeout of 5 seconds.

The wait command can be used to wait for a file to appear
during the boot process, for example a disk or other device,
and is a better alternative than an unconditional sleep.

Change-Id: Ia11d8419c0ea84deec0e1bf5adfca5bc76ff38b7
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>