r2dplus: select rtl8139 driver in defconfig

CONFIG_RTL8139 was moved to a board defconfig by a commit 86e9dc86b1
("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 adad96e60d ("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>

configs/r2dplus_defconfig

@@ -5,5 +5,7 @@ CONFIG_BOOTDELAY=-1
 CONFIG_CMD_PING=y
 CONFIG_CMD_CACHE=y
 CONFIG_CMD_EXT2=y
+CONFIG_NETDEVICES=y
+CONFIG_RTL8139=y
 CONFIG_PCI=y
 CONFIG_USE_PRIVATE_LIBGCC=y