complete: add empty complete support ...

for cpuinfo, clear, dhcp, false, login, lsmod, meminfo, passwd, pwd, reginfo,
reset, true, usb, version

for mach-imx and mach-mxs: dump_clocks
for u_serial: mycdev

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>

usb command: by default scan only once for USB devices ...

We normally do not expect USB devices to be hotplugged. Instead of
rescanning each time the usb command is called, scan only once.
This makes the usb command safe for being called multiple times
without making already registered USB devices reinitialized. To
really scan multiple times a '-f' option is introduced.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

commands: remove struct command pointer from commands ...

This is unused in all commands and thus can be removed.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

commands: seperate usb command from usb core ...

This patch makes the USB command optional and makes usb_rescan a
global function. This way we can use USB in noninteractive
environments.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>