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/turris_mox_defconfig b/configs/turris_mox_defconfig
index c1bd1de..018ec50 100644
--- a/configs/turris_mox_defconfig
+++ b/configs/turris_mox_defconfig
@@ -28,6 +28,7 @@
 CONFIG_ARCH_EARLY_INIT_R=y
 CONFIG_LAST_STAGE_INIT=y
 CONFIG_MISC_INIT_R=y
+CONFIG_SYS_MAXARGS=32
 CONFIG_CMD_SHA1SUM=y
 CONFIG_CMD_CLK=y
 # CONFIG_CMD_FLASH is not set