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/imx6ul_geam_mmc_defconfig b/configs/imx6ul_geam_mmc_defconfig
index c59b56d..35610c6 100644
--- a/configs/imx6ul_geam_mmc_defconfig
+++ b/configs/imx6ul_geam_mmc_defconfig
@@ -7,6 +7,7 @@
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y
+# CONFIG_CMD_BMODE is not set
 CONFIG_DEFAULT_DEVICE_TREE="imx6ul-geam-kit"
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y