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/M5373EVB_defconfig b/configs/M5373EVB_defconfig
index 1d832ba..c59ba94 100644
--- a/configs/M5373EVB_defconfig
+++ b/configs/M5373EVB_defconfig
@@ -23,6 +23,7 @@
 CONFIG_ENV_ADDR=0x4000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SYS_FAULT_ECHO_LINK_DOWN=y
+CONFIG_SYS_RX_ETH_BUFFER=8
 CONFIG_UDP_CHECKSUM=y
 CONFIG_SYS_I2C_LEGACY=y
 CONFIG_SYS_I2C_FSL=y