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/ap121_defconfig b/configs/ap121_defconfig
index afe6c96..9ce2288 100644
--- a/configs/ap121_defconfig
+++ b/configs/ap121_defconfig
@@ -5,6 +5,8 @@
 CONFIG_DEBUG_UART=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_BOOTDELAY=3
+CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS="console=ttyS0,115200 root=/dev/mtdblock2 rootfstype=squashfs"
 CONFIG_DISPLAY_CPUINFO=y
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SYS_PROMPT="ap121 # "