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>

net/Kconfig

@@ -160,3 +160,12 @@ config BOOTP_SERVERIP
 	  bootp and tftp.
 
 endif   # if NET
+
+config SYS_RX_ETH_BUFFER
+	int "Number of receive packet buffers"
+	default 4
+	help
+	  Defines the number of Ethernet receive buffers. On some Ethernet
+	  controllers it is recommended to set this value to 8 or even higher,
+	  since all buffers can be full shortly after enabling the interface on
+	  high Ethernet traffic.