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_usbhost_boot_defconfig b/configs/am43xx_evm_usbhost_boot_defconfig
index 144ab21..be56573 100644
--- a/configs/am43xx_evm_usbhost_boot_defconfig
+++ b/configs/am43xx_evm_usbhost_boot_defconfig
@@ -54,6 +54,7 @@
 CONFIG_VERSION_VARIABLE=y
 CONFIG_NET_RETRY_COUNT=10
 CONFIG_BOOTP_SEND_HOSTNAME=y
+CONFIG_SYS_RX_ETH_BUFFER=64
 CONFIG_DM=y
 CONFIG_REGMAP=y
 CONFIG_SPL_REGMAP=y