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/sh7752evb_defconfig b/configs/sh7752evb_defconfig
index b23d39c..721247a 100644
--- a/configs/sh7752evb_defconfig
+++ b/configs/sh7752evb_defconfig
@@ -3,6 +3,8 @@
 CONFIG_TARGET_SH7752EVB=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
+CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS="console=ttySC2,115200 root=/dev/nfs ip=dhcp"
 CONFIG_VERSION_VARIABLE=y
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set