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/apalis_imx6_defconfig b/configs/apalis_imx6_defconfig
index ea4ad27..718623d 100644
--- a/configs/apalis_imx6_defconfig
+++ b/configs/apalis_imx6_defconfig
@@ -42,6 +42,7 @@
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_USB_SDP_SUPPORT=y
 CONFIG_SYS_PROMPT="Apalis iMX6 # "
+CONFIG_SYS_MAXARGS=48
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_XIMG is not set