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>

cmd/Makefile

@@ -8,6 +8,7 @@ ifndef CONFIG_SPL_BUILD
 obj-y += boot.o
 obj-$(CONFIG_CMD_BOOTM) += bootm.o
 obj-y += help.o
+obj-y += panic.o
 obj-y += version.o
 
 # command