net: lwip: add Kconfig option to show ICMP unreachable errors

net: lwip: add Kconfig option to show ICMP unreachable errors

Add Kconfig symbol LWIP_ICMP_SHOW_UNREACH which, when enabled, prints a
message to the console upon reception of ICMP unreachable messages. For
example:

 $ make qemu_arm64_lwip_defconfig
 $ qemu-system-aarch64 -M virt -cpu max -nographic -bios u-boot.bin
 [...]
 => dhcp
 DHCP client bound to address 10.0.2.15 (0 ms)
 => tftp 192.168.0.100:69:Image
 Using virtio-net#32 device
 TFTP from server 192.168.0.100; our IP address is 10.0.2.15
 Filename 'Image'.
 Load address: 0x40200000
 Loading: ICMP destination unreachable (host unreachable) from 192.168.0.16
 Timeout!
 => tftp 192.168.0.16:69:Image
 Using virtio-net#32 device
 TFTP from server 192.168.0.16; our IP address is 10.0.2.15
 Filename 'Image'.
 Load address: 0x40200000
 Loading: ICMP destination unreachable (port unreachable) from 192.168.0.16
 Timeout!
 =>

Submitted upstream as https://github.com/lwip-tcpip/lwip/pull/73.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>