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/imx8mn_beacon_2g_defconfig b/configs/imx8mn_beacon_2g_defconfig
index 5f116b7..b1f5cb4 100644
--- a/configs/imx8mn_beacon_2g_defconfig
+++ b/configs/imx8mn_beacon_2g_defconfig
@@ -28,6 +28,8 @@
 CONFIG_SPL_LOAD_FIT=y
 CONFIG_SPL_FIT_GENERATOR="arch/arm/mach-imx/mkimage_fit_atf.sh"
 CONFIG_OF_SYSTEM_SETUP=y
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="mmc dev ${mmcdev}; if mmc rescan; then if run loadbootscript; then run bootscript; else if run loadimage; then run mmcboot; else run netboot; fi; fi; else booti ${loadaddr} - ${fdt_addr}; fi"
 CONFIG_DEFAULT_FDT_FILE="imx8mn-beacon-kit.dtb"
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_SPL_BOARD_INIT=y