common: Move CONFIG_BOOTARGS to Kconfig

Also introduce CONFIG_USE_BOOTARGS option so we can control if
CONFIG_BOOTARGS defined at all.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
[trini: Resync r8a779[56]_ulcb, various ls10xx targets]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/M54418TWR_serial_rmii_defconfig b/configs/M54418TWR_serial_rmii_defconfig
index bf0b34f..debe959 100644
--- a/configs/M54418TWR_serial_rmii_defconfig
+++ b/configs/M54418TWR_serial_rmii_defconfig
@@ -3,6 +3,8 @@
 CONFIG_TARGET_M54418TWR=y
 CONFIG_SYS_EXTRA_OPTIONS="CF_SBF,SYS_SERIAL_BOOT,SYS_INPUT_CLKSRC=50000000"
 CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS="root=/dev/nfs rw nfsroot=192.168.1.1:/tftpboot/192.168.1.2  ip=192.168.1.2:192.168.1.1:192.168.1.1: 255.255.255.0::eth0:off:rw console=ttyS0,115200"
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="-> "