net: Move CONFIG_RTL8139 to Kconfig

Introduce CONFIG_RTL8139 in Kconfig and move over boards' defconfig
to use that.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
[trini: Fixup MPC8641HPCN* and r2dplus configs]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/r2dplus_defconfig b/configs/r2dplus_defconfig
index cca0156..ddb184b 100644
--- a/configs/r2dplus_defconfig
+++ b/configs/r2dplus_defconfig
@@ -1,4 +1,5 @@
 CONFIG_SH=y
 CONFIG_TARGET_R2DPLUS=y
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_RTL8139=y
 CONFIG_USE_PRIVATE_LIBGCC=y