kernel: enable Multi-Path TCP for !SMALL_FLASH targets

kernel: enable Multi-Path TCP for !SMALL_FLASH targets

Expose Kernel's CONFIG_MPTCP option and enable it by default for
!SMALL_FLASH targets.

The idea behind enabling it by default is to allow users of the binary
distribution to make use of MPTCP tunneling for link aggregation.

Using MPTCP for link aggregation is an often discussed topic in the
forum and there is even a whole OpenWrt fork (MPTCPRouter) just for that.

Enabling the kernel-side of the story by default will allow using MPTCP
on vanilla OpenWrt without having to build anything from source.

See also https://openwrt.org/docs/guide-user/network/mptcp

Signed-off-by: Daniel Golle <daniel@makrotopia.org>