Convert CONFIG_SYS_RX_ETH_BUFFER to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_RX_ETH_BUFFER

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/pic32mzdask_defconfig b/configs/pic32mzdask_defconfig
index d5008c4..60e0d47 100644
--- a/configs/pic32mzdask_defconfig
+++ b/configs/pic32mzdask_defconfig
@@ -34,6 +34,7 @@
 CONFIG_ARP_TIMEOUT=500
 CONFIG_NET_RETRY_COUNT=20
 CONFIG_NET_RANDOM_ETHADDR=y
+CONFIG_SYS_RX_ETH_BUFFER=8
 CONFIG_CLK=y
 CONFIG_MMC=y
 CONFIG_MMC_SDHCI=y