Convert CONFIG_ARP_TIMEOUT to Kconfig

This converts the following to Kconfig:
   CONFIG_ARP_TIMEOUT

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/riotboard_defconfig b/configs/riotboard_defconfig
index 5633270..bdfe51c 100644
--- a/configs/riotboard_defconfig
+++ b/configs/riotboard_defconfig
@@ -44,6 +44,7 @@
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SYS_MMC_ENV_DEV=2
+CONFIG_ARP_TIMEOUT=200
 CONFIG_DM=y
 CONFIG_BOUNCE_BUFFER=y
 CONFIG_SYS_I2C_LEGACY=y