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/pico-pi-imx6ul_defconfig b/configs/pico-pi-imx6ul_defconfig
index 31f37da..6bc8d5d 100644
--- a/configs/pico-pi-imx6ul_defconfig
+++ b/configs/pico-pi-imx6ul_defconfig
@@ -29,6 +29,7 @@
 CONFIG_SPL_USB_HOST=y
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_USB_SDP_SUPPORT=y
+CONFIG_SYS_MAXARGS=32
 CONFIG_CMD_BOOTMENU=y
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_DFU=y