net: if: Handle new address type of NET_ADDR_OVERRIDABLE

The meaning of this address type is the same as NET_ADDR_MANUAL,
but with a provision that DHCP can override such an address.
It's intended for the usecase when there's a default static
configuration for when DHCP is not available, but DHCP should
override it.

Before going to add another address type, there was an attempt
to repurpose TENTATIVE address state, but it doesn't work as
expected, as indeed, all existing address types/states already
have clearly semantics, and it makes sense to just another
address type to avoid confusion and unexpected behavior.

Fixes: #5696

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>