cmd: Add the 'btrsubvol' command to list BTRFS subvolumes

Signed-off-by: Marek Behun <marek.behun@nic.cz>

 create mode 100644 cmd/btrfs.c
diff --git a/cmd/Makefile b/cmd/Makefile
index 2a5b8ce..2b0444d 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -27,6 +27,7 @@
 obj-$(CONFIG_CMD_BOOTSTAGE) += bootstage.o
 obj-$(CONFIG_CMD_BOOTZ) += bootz.o
 obj-$(CONFIG_CMD_BOOTI) += booti.o
+obj-$(CONFIG_CMD_BTRFS) += btrfs.o
 obj-$(CONFIG_CMD_CACHE) += cache.o
 obj-$(CONFIG_CMD_CBFS) += cbfs.o
 obj-$(CONFIG_CMD_CLK) += clk.o