Convert CONFIG_SYS_MAXARGS to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MAXARGS

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/mt7629_rfb_defconfig b/configs/mt7629_rfb_defconfig
index 4d47b47..87e5737 100644
--- a/configs/mt7629_rfb_defconfig
+++ b/configs/mt7629_rfb_defconfig
@@ -28,6 +28,7 @@
 CONFIG_SPL_WATCHDOG=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
+CONFIG_SYS_MAXARGS=8
 CONFIG_CMD_BOOTMENU=y
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_XIMG is not set