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/vexpress_aemv8r_defconfig b/configs/vexpress_aemv8r_defconfig
index a1c5d88..6b470f5 100644
--- a/configs/vexpress_aemv8r_defconfig
+++ b/configs/vexpress_aemv8r_defconfig
@@ -11,5 +11,6 @@
 CONFIG_BOOTARGS="console=ttyAMA0 earlycon=pl011,0x9c090000 rootfstype=ext4 root=/dev/vda2 rw rootwait"
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SYS_PROMPT="VExpress64# "
+CONFIG_SYS_MAXARGS=64
 # CONFIG_MMC is not set
 CONFIG_VIRTIO_MMIO=y