Fix build issues when building for RP2040 from pico-examples/pico-sdk.

* include_guard requires GLOBAL as family.cmake is included in multiple non child places
* the following recently added check is suprfluous (family_configure_host_example for rp2040 should do this already),
  and breaks if pico_pio_usb is not avaialble, so i have removed
   # Add pico-pio-usb for rp2040 since user can choose to run on bit-banging host
   if(FAMILY STREQUAL "rp2040")
      family_add_pico_pio_usb(${PROJECT})
   endif()
* added new familt_example_missing_dependency functino to print missing dependency warning, so
  pico-examples can override it to be less in your face, and also more contextual to pico-examples

examples/device/net_lwip_webserver/CMakeLists.txt

@@ -2,15 +2,15 @@ cmake_minimum_required(VERSION 3.17)
 
 include(${CMAKE_CURRENT_LIST_DIR}/../../../hw/bsp/family_support.cmake)
 
-set(LWIP ${TOP}/lib/lwip)
-if (NOT EXISTS ${LWIP}/src)
-  MESSAGE(WARNING "lib/lwip submodule not found, please run 'python tools/get_deps.py lib/lwip' to fetch it")
-  return()
-endif()
-
 # gets PROJECT name for the example (e.g. <BOARD>-<DIR_NAME>)
 family_get_project_name(PROJECT ${CMAKE_CURRENT_LIST_DIR})
 
+set(LWIP ${TOP}/lib/lwip)
+if (NOT EXISTS ${LWIP}/src)
+  family_example_missing_dependency(${PROJECT} "lib/lwip")
+  return()
+endif()
+
 project(${PROJECT} C CXX ASM)
 
 # Checks this example is valid for the family and initializes the project