net: Move CONFIG_RTL8169 to Kconfig

Introduce CONFIG_RTL8169 in Kconfig and move over boards' defconfig
to use that.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Stephen Warren <swaren@nvidia.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/configs/beaver_defconfig b/configs/beaver_defconfig
index fcfbf65..9e6a5c3 100644
--- a/configs/beaver_defconfig
+++ b/configs/beaver_defconfig
@@ -15,6 +15,7 @@
 # CONFIG_CMD_NFS is not set
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_WINBOND=y
+CONFIG_RTL8169=y
 CONFIG_PCI_TEGRA=y
 CONFIG_SYS_NS16550=y
 CONFIG_TEGRA20_SLINK=y