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/host/msc_file_explorer/CMakeLists.txt

@@ -38,8 +38,3 @@ target_include_directories(${PROJECT} PUBLIC
 # Configure compilation flags and libraries for the example without RTOS.
 # See the corresponding function in hw/bsp/FAMILY/family.cmake for details.
 family_configure_host_example(${PROJECT} noos)
-
-# 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()