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/mx6sllevk_plugin_defconfig b/configs/mx6sllevk_plugin_defconfig
index 8a7a01b..41ecf92 100644
--- a/configs/mx6sllevk_plugin_defconfig
+++ b/configs/mx6sllevk_plugin_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_MX6=y
 CONFIG_TARGET_MX6SLLEVK=y
 CONFIG_USE_IMXIMG_PLUGIN=y
+# CONFIG_CMD_BMODE is not set
 CONFIG_DEFAULT_DEVICE_TREE="imx6sll-evk"
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6sllevk/imximage.cfg"
 CONFIG_BOOTDELAY=3