Convert CONFIG_USE_BOOTCOMMAND et al to Kconfig

This converts the following to Kconfig:
   CONFIG_USE_BOOTCOMMAND
   CONFIG_BOOTCOMMAND
   CONFIG_RAMBOOTCOMMAND
   CONFIG_NFSBOOTCOMMAND

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/opos6uldev_defconfig b/configs/opos6uldev_defconfig
index c52d2a5..971ae90 100644
--- a/configs/opos6uldev_defconfig
+++ b/configs/opos6uldev_defconfig
@@ -23,6 +23,8 @@
 CONFIG_BOOTDELAY=5
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttymxc0,115200"
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="run emmcboot"
 CONFIG_USE_PREBOOT=y
 CONFIG_PREBOOT="run check_env"
 CONFIG_DEFAULT_FDT_FILE="imx6ul-opos6uldev.dtb"