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/bcm_ns3_defconfig b/configs/bcm_ns3_defconfig
index 1305434..86ef5e3 100644
--- a/configs/bcm_ns3_defconfig
+++ b/configs/bcm_ns3_defconfig
@@ -14,6 +14,8 @@
 CONFIG_LEGACY_IMAGE_FORMAT=y
 CONFIG_SUPPORT_RAW_INITRD=y
 CONFIG_OF_BOARD_SETUP=y
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="run flash_pending_rfs_imgs;run fastboot_nitro && run bootcmd_mmc_fits || run bootcmd_usb || run bootcmd_pxe"
 CONFIG_LOGLEVEL=7
 CONFIG_SILENT_CONSOLE=y
 CONFIG_SILENT_U_BOOT_ONLY=y