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/evb-ast2500_defconfig b/configs/evb-ast2500_defconfig
index ae650fc..95ddc9c 100644
--- a/configs/evb-ast2500_defconfig
+++ b/configs/evb-ast2500_defconfig
@@ -5,6 +5,8 @@
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_DEFAULT_DEVICE_TREE="ast2500-evb"
 CONFIG_ENV_IS_NOWHERE=y
+CONFIG_USE_BOOTARGS=y
+CONFIG_BOOTARGS="console=ttyS4,115200n8 root=/dev/ram rw"
 CONFIG_PRE_CONSOLE_BUFFER=y
 CONFIG_PRE_CON_BUF_ADDR=0x1e720000
 # CONFIG_DISPLAY_CPUINFO is not set