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/MPC8641HPCN_36BIT_defconfig b/configs/MPC8641HPCN_36BIT_defconfig
index 98298b7..cfe26ea 100644
--- a/configs/MPC8641HPCN_36BIT_defconfig
+++ b/configs/MPC8641HPCN_36BIT_defconfig
@@ -5,5 +5,6 @@
 CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_SYS_EXTRA_OPTIONS="PHYS_64BIT"
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_RTL8139=y
 CONFIG_SYS_NS16550=y
 CONFIG_OF_LIBFDT=y