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/integratorap_cm926ejs_defconfig b/configs/integratorap_cm926ejs_defconfig
index 52973da..af0d73b 100644
--- a/configs/integratorap_cm926ejs_defconfig
+++ b/configs/integratorap_cm926ejs_defconfig
@@ -22,6 +22,7 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_BOOTP_BOOTFILESIZE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_SYS_RX_ETH_BUFFER=8
 # CONFIG_MMC is not set
 CONFIG_MTD_NOR_FLASH=y
 CONFIG_FLASH_CFI_DRIVER=y