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/pxm2_defconfig b/configs/pxm2_defconfig
index c205161..23bb593 100644
--- a/configs/pxm2_defconfig
+++ b/configs/pxm2_defconfig
@@ -45,6 +45,7 @@
 CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot# "
+CONFIG_SYS_MAXARGS=32
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_DFU=y
 # CONFIG_CMD_FLASH is not set