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/pico-imx6ul_defconfig b/configs/pico-imx6ul_defconfig
index a2a303c..d18a96e 100644
--- a/configs/pico-imx6ul_defconfig
+++ b/configs/pico-imx6ul_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_MX6=y
 CONFIG_TARGET_PICO_IMX6UL=y
+# CONFIG_CMD_BMODE is not set
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/technexion/pico-imx6ul/imximage.cfg"
 CONFIG_BOOTDELAY=3
 CONFIG_DEFAULT_FDT_FILE="imx6ul-pico-hobbit.dtb"