Make "usage" messages more helpful.

In case of incorrect command invocations U-Boot used to print pretty
useless "usage" messages, for example:

	=> nand markbad
	Usage:
	nand - NAND sub-system

In the result, the user would have to run the "help" command to get
the (available) information about correct command usage. Change this,
so that this information gets always printed.

Note that this changes the user interface of all commands, but
hopefully to the better.

Signed-off-by: Wolfgang Denk <wd@denx.de>