arm: kirkwood: Enable bootstd for Zyxel NSA310S board

Enable bootstd for Zyxel NSA310S board, and remove distroboot.

Signed-off-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/configs/nsa310s_defconfig b/configs/nsa310s_defconfig
index ff489562..038ef42 100644
--- a/configs/nsa310s_defconfig
+++ b/configs/nsa310s_defconfig
@@ -19,7 +19,8 @@
 CONFIG_IDENT_STRING="\nZyXEL NSA310S/320S 1/2-Bay Power Media Server"
 CONFIG_SYS_LOAD_ADDR=0x800000
 CONFIG_DEBUG_UART=y
-CONFIG_DISTRO_DEFAULTS=y
+CONFIG_BOOTSTD_FULL=y
+CONFIG_BOOTSTD_DEFAULTS=y
 CONFIG_BOOTDELAY=3
 CONFIG_USE_PREBOOT=y
 # CONFIG_DISPLAY_BOARDINFO is not set