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/colibri_imx6_defconfig b/configs/colibri_imx6_defconfig
index 4f38d5c..5533812 100644
--- a/configs/colibri_imx6_defconfig
+++ b/configs/colibri_imx6_defconfig
@@ -41,6 +41,7 @@
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_USB_SDP_SUPPORT=y
 CONFIG_SYS_PROMPT="Colibri iMX6 # "
+CONFIG_SYS_MAXARGS=48
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_XIMG is not set