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/axs101_defconfig b/configs/axs101_defconfig
index 8d36371..edaa992 100644
--- a/configs/axs101_defconfig
+++ b/configs/axs101_defconfig
@@ -7,6 +7,8 @@
 CONFIG_ENV_FAT_INTERFACE="mmc"
 CONFIG_ENV_FAT_DEVICE_AND_PART="0:1"
 CONFIG_BOOTDELAY=3
+CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS="console=ttyS3,115200n8"
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SYS_PROMPT="AXS# "
 # CONFIG_CMD_IMLS is not set