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/10m50_defconfig b/configs/10m50_defconfig
index 0a8b30a..517c3b0 100644
--- a/configs/10m50_defconfig
+++ b/configs/10m50_defconfig
@@ -28,6 +28,7 @@
 CONFIG_VERSION_VARIABLE=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SYS_FAULT_ECHO_LINK_DOWN=y
+CONFIG_SYS_RX_ETH_BUFFER=0
 CONFIG_ALTERA_PIO=y
 CONFIG_MISC=y
 CONFIG_ALTERA_SYSID=y