kernel/hack-5.4: make UDP tunneling user-selectable

kernel/hack-5.4: make UDP tunneling user-selectable

UDP tunneling support isn't user-selectable, but it's required by WireGuard
which is, for the time being, an out-of-tree module. We currently work around
this issue by selecting an unrelated module which depends on UDP tunnelling
(VXLAN). This is inconvenient, as it implies this unrelated module needs to be
built-in when doing a monolithic build.

Fix this inconvenience by making UDP tunneling user-selectable in the kernel
configuration.

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>