Convert CONFIG_CMD_BMODE to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_BMODE

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Make this default y and depend on mx5/6/7]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/mx6sxsabreauto_defconfig b/configs/mx6sxsabreauto_defconfig
index b546740..f9c2af9 100644
--- a/configs/mx6sxsabreauto_defconfig
+++ b/configs/mx6sxsabreauto_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_MX6=y
 CONFIG_TARGET_MX6SXSABREAUTO=y
+# CONFIG_CMD_BMODE is not set
 CONFIG_DEFAULT_DEVICE_TREE="imx6sx-sabreauto"
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6sxsabreauto/imximage.cfg"
 CONFIG_BOOTDELAY=3