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/am43xx_evm_qspiboot_defconfig b/configs/am43xx_evm_qspiboot_defconfig
index 31a2de5..6bbb962 100644
--- a/configs/am43xx_evm_qspiboot_defconfig
+++ b/configs/am43xx_evm_qspiboot_defconfig
@@ -40,6 +40,7 @@
 CONFIG_VERSION_VARIABLE=y
 CONFIG_NET_RETRY_COUNT=10
 CONFIG_BOOTP_SEND_HOSTNAME=y
+CONFIG_SYS_RX_ETH_BUFFER=64
 CONFIG_DM=y
 CONFIG_DFU_MMC=y
 CONFIG_DFU_RAM=y