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/spear300_usbtty_defconfig b/configs/spear300_usbtty_defconfig
index c5d4761..c9cd7d0 100644
--- a/configs/spear300_usbtty_defconfig
+++ b/configs/spear300_usbtty_defconfig
@@ -4,6 +4,8 @@
 CONFIG_SYS_EXTRA_OPTIONS="SPEAR300,USBTTY"
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_BOOTDELAY=-1
+CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS="console=ttyAMA0,115200 mem=128M root=/dev/mtdblock3 rootfstype=jffs2"
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_CMD_NAND=y