cmd: add a panic command

Even in boot scripts it may be needed to "panic" when all options
are exhausted and the device specification specifies hanging
instead of resetting the board.

So add a new panic command that just wraps around the core panic
call in U-Boot and can take an optional message.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>