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/eb_cpu5282_internal_defconfig b/configs/eb_cpu5282_internal_defconfig
index a91d578..4adc056 100644
--- a/configs/eb_cpu5282_internal_defconfig
+++ b/configs/eb_cpu5282_internal_defconfig
@@ -25,6 +25,7 @@
 CONFIG_CMD_DATE=y
 CONFIG_ENV_ADDR=0xFF040000
 CONFIG_SYS_FAULT_ECHO_LINK_DOWN=y
+CONFIG_SYS_RX_ETH_BUFFER=8
 CONFIG_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_FSL=y
 CONFIG_SYS_FSL_I2C_OFFSET=0x300