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/opos6uldev_defconfig b/configs/opos6uldev_defconfig
index b9dab54..b921367 100644
--- a/configs/opos6uldev_defconfig
+++ b/configs/opos6uldev_defconfig
@@ -12,6 +12,8 @@
 CONFIG_DEFAULT_DEVICE_TREE="imx6ul-opos6uldev"
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_BOOTDELAY=5
+CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS="console=ttymxc0,115200"
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y
 CONFIG_DEFAULT_FDT_FILE="imx6ul-opos6uldev.dtb"