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 <[email protected]>

config/Config-kernel.in

@@ -1204,6 +1204,20 @@ config KERNEL_PAGE_POOL_STATS
 	bool "Page pool stats support"
 	depends on KERNEL_PAGE_POOL
 
+config KERNEL_MPTCP
+	bool "Multi-Path TCP support"
+	default y if !SMALL_FLASH
+	help
+	   Select this option to enable support for Multi-Path TCP.
+	   Increases the compressed kernel size by ~214kB (as of Linux 6.6).
+
+if KERNEL_IPV6
+config KERNEL_MPTCP_IPV6
+	bool "IPv6 support for Multipath TCP"
+	depends on KERNEL_MPTCP
+	default KERNEL_MPTCP
+endif
+
 #
 # NFS related symbols
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