r2dplus: select rtl8139 driver in defconfig

r2dplus: select rtl8139 driver in defconfig

CONFIG_RTL8139 was moved to a board defconfig by a commit 86e9dc86b1a2
("net: Move CONFIG_RTL8139 to Kconfig"), however it was done
incorrectly due to a missing CONFIG_NETDEVICES selection, thus
virtually it was just a removal of the driver compilation.

As an unlucky consequence the option was completely removed by a purge
commit adad96e60d0e ("configs: Re-sync HUSH options"), restore the
driver inclusion back.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>