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/r7780mp_defconfig b/configs/r7780mp_defconfig
index b2596a4..81c324f 100644
--- a/configs/r7780mp_defconfig
+++ b/configs/r7780mp_defconfig
@@ -2,6 +2,8 @@
 CONFIG_TARGET_R7780MP=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=3
+CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS="console=ttySC0,115200"
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_BOOTD is not set