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/M5235EVB_defconfig b/configs/M5235EVB_defconfig
index 22d4f43..d4f32e4 100644
--- a/configs/M5235EVB_defconfig
+++ b/configs/M5235EVB_defconfig
@@ -28,6 +28,7 @@
 CONFIG_USE_BOOTFILE=y
 CONFIG_BOOTFILE="u-boot.bin"
 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