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/mt8516_pumpkin_defconfig b/configs/mt8516_pumpkin_defconfig
index b073579..53ea050 100644
--- a/configs/mt8516_pumpkin_defconfig
+++ b/configs/mt8516_pumpkin_defconfig
@@ -15,6 +15,8 @@
 CONFIG_SYS_LOAD_ADDR=0x4c000000
 CONFIG_FIT=y
 # CONFIG_ARCH_FIXUP_FDT_MEMORY is not set
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="run distro_bootcmd"
 CONFIG_DEFAULT_FDT_FILE="mt8516-pumpkin"
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_BOARD_LATE_INIT=y