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/imx6ul_isiot_emmc_defconfig b/configs/imx6ul_isiot_emmc_defconfig
index 256d732..e701d06 100644
--- a/configs/imx6ul_isiot_emmc_defconfig
+++ b/configs/imx6ul_isiot_emmc_defconfig
@@ -25,6 +25,8 @@
 CONFIG_LEGACY_IMAGE_FORMAT=y
 CONFIG_SUPPORT_RAW_INITRD=y
 CONFIG_BOOTDELAY=3
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="run $modeboot"
 CONFIG_SPL_RAW_IMAGE_SUPPORT=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_HUSH_PARSER=y