DHCP indicate new config if there's a packet buffer provided

\# Description

If the user provides a buffer in which to store the packets, then the
contents of the received packet will be buffered and included in the
returned Config when the DHCP connection is made. However, it isn't
included in the Config struct until the value is returned to the user,
so the equality check for whether to call `config_changed` disregards
any additional information in the buffer beyond what this library
parses.

For my purposes, I need access to the contents of the buffer when they
change as a result of a new packet, even if everything else is the same.
Since two packets will almost certainly not be the same thanks to the
magic cookie (unless the packet gets duplicated on the network, which is
an acceptably low risk for my use of smoltcp), an acceptable option for
my uses is to just always return the new configuration when a packet is
received (gated on whether a buffer is provided to return the packet
into).

\# Alternatives

While this approach is the easiest and best for my uses, I can think of
the following alternatives which would also work and might be prefered
for other use-cases:
 * Allow the user to specify whether they wish to receive all packets
   instead of opting all users who provide a buffer into this behavior
 * Allow the user to provide a closure which compares the old and new
   packets and returns true if this represents a new config which should
   be returned.
 * Compare the old packet to the new packet (either byte-by-byte or
   looking at the provided options) and only return a new config if
   differences are found.
\# Verification

In my setup, I was seeing bugs that were caused by smoltcp not exposing
the config when the only changes were in the additional options that I
want to use but which smoltcp doesn't use directly. Using this branch
instead of the main release fixed those bugs and I was able to verify
that it behaves the way I expected. I think this verification, along
with CI tests passing, should be sufficient for verifying this PR.

src/socket/dhcpv4.rs

@@ -361,8 +361,17 @@ impl<'a> Socket<'a> {
                 ) {
                     state.renew_at = renew_at;
                     state.expires_at = expires_at;
+                    // The `receive_packet_buffer` field isn't populated until
+                    // the client asks for the state, but receiving any packet
+                    // will change it, so we indicate that the config has
+                    // changed every time if the receive packet buffer is set,
+                    // but we only write changes to the rest of the config now.
+                    let config_changed =
+                        state.config != config || self.receive_packet_buffer.is_some();
                     if state.config != config {
                         state.config = config;
+                    }
+                    if config_changed {
                         self.config_changed();
                     }
                 }