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/comtrend_ar5387un_ram_defconfig b/configs/comtrend_ar5387un_ram_defconfig
index ea7d7b9..5a94448 100644
--- a/configs/comtrend_ar5387un_ram_defconfig
+++ b/configs/comtrend_ar5387un_ram_defconfig
@@ -40,6 +40,7 @@
 # CONFIG_CMD_SLEEP is not set
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_NET_RANDOM_ETHADDR=y
+CONFIG_SYS_RX_ETH_BUFFER=6
 # CONFIG_DM_DEVICE_REMOVE is not set
 CONFIG_BCM6348_IUDMA=y
 CONFIG_LED=y