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/rcar3_salvator-x_defconfig b/configs/rcar3_salvator-x_defconfig
index 2b2c273..adbfbf2 100644
--- a/configs/rcar3_salvator-x_defconfig
+++ b/configs/rcar3_salvator-x_defconfig
@@ -13,6 +13,8 @@
 CONFIG_FIT=y
 CONFIG_SUPPORT_RAW_INITRD=y
 CONFIG_USE_BOOTARGS=y
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="tftp 0x48080000 Image; tftp 0x48000000 Image-r8a77950-salvator-x.dtb; booti 0x48080000 - 0x48000000"
 CONFIG_DEFAULT_FDT_FILE="r8a77950-salvator-x.dtb"
 CONFIG_UPDATE_TFTP=y
 CONFIG_HUSH_PARSER=y