Convert CONFIG_BOOTARGS to Kconfig

This converts the following to Kconfig:
   CONFIG_BOOTARGS

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/wb50n_defconfig b/configs/wb50n_defconfig
index 1b444f0..b2f490a 100644
--- a/configs/wb50n_defconfig
+++ b/configs/wb50n_defconfig
@@ -15,6 +15,8 @@
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="SAMA5D3,SYS_USE_NANDFLASH"
 CONFIG_BOOTDELAY=3
+CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS="rw rootfstype=ubifs ubi.mtd=6 root=ubi0:rootfs"
 CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y