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/variscite_dart6ul_defconfig b/configs/variscite_dart6ul_defconfig
index 53a49cc..bcca038 100644
--- a/configs/variscite_dart6ul_defconfig
+++ b/configs/variscite_dart6ul_defconfig
@@ -17,7 +17,7 @@
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3
-# CONFIG_USE_BOOTCOMMAND is not set
+CONFIG_BOOTCOMMAND="run mmc_mmc_fit"
 CONFIG_SPL_USB_HOST=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_CMD_DM=y