cmd: button: add a new 'button' command

Adds a command 'button' that provides the list of buttons
supported by the board, and the state of a button.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
diff --git a/cmd/Makefile b/cmd/Makefile
index 7952138..6e0086b 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -33,6 +33,7 @@
 obj-$(CONFIG_CMD_BOOTZ) += bootz.o
 obj-$(CONFIG_CMD_BOOTI) += booti.o
 obj-$(CONFIG_CMD_BTRFS) += btrfs.o
+obj-$(CONFIG_CMD_BUTTON) += button.o
 obj-$(CONFIG_CMD_CACHE) += cache.o
 obj-$(CONFIG_CMD_CBFS) += cbfs.o
 obj-$(CONFIG_CMD_CLK) += clk.o