cmd_boot: Add a poweroff command

Add a 'poweroff' command to boot commands, this only gets enabled if the
board Kconfig does a "select CMD_POWEROFF".

Signed-off-by: Michael van Slingerland <michael@deviousops.nl>
[hdegoede@redhat.com: Make the cmd conditional on a CMD_POWEROFF Kconfig]
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
diff --git a/common/Kconfig b/common/Kconfig
index ccf5475..9d446bf 100644
--- a/common/Kconfig
+++ b/common/Kconfig
@@ -184,6 +184,9 @@
 	help
 	  Extract a part of a multi-image.
 
+config CMD_POWEROFF
+	bool
+
 endmenu
 
 menu "Environment commands"