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/ls1028aqds_tfa_SECURE_BOOT_defconfig b/configs/ls1028aqds_tfa_SECURE_BOOT_defconfig
index af65bca..94a1bd3 100644
--- a/configs/ls1028aqds_tfa_SECURE_BOOT_defconfig
+++ b/configs/ls1028aqds_tfa_SECURE_BOOT_defconfig
@@ -43,6 +43,7 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_NETCONSOLE=y
+CONFIG_SYS_RX_ETH_BUFFER=8
 CONFIG_DM=y
 CONFIG_SATA=y
 CONFIG_SCSI_AHCI=y