hello_world: fix build guards to allow host build, and improve missing tinyusb message

Author: kilograham

hello_world/CMakeLists.txt

@@ -1,4 +1,2 @@
-if (NOT PICO_NO_HARDWARE)
-    add_subdirectory(usb)
-    add_subdirectory(serial)
-endif ()
+add_subdirectory(serial)
+add_subdirectory(usb)

hello_world/usb/CMakeLists.txt

@@ -1,16 +1,20 @@
-add_executable(hello_usb
-        hello_usb.c
-        )
+if (TARGET tinyusb_device)
+    add_executable(hello_usb
+            hello_usb.c
+            )
 
-# Pull in our pico_stdlib which aggregates commonly used features
-target_link_libraries(hello_usb pico_stdlib)
+    # Pull in our pico_stdlib which aggregates commonly used features
+    target_link_libraries(hello_usb pico_stdlib)
 
-# enable usb output, disable uart output
-pico_enable_stdio_usb(hello_usb 1)
-pico_enable_stdio_uart(hello_usb 0)
+    # enable usb output, disable uart output
+    pico_enable_stdio_usb(hello_usb 1)
+    pico_enable_stdio_uart(hello_usb 0)
 
-# create map/bin/hex/uf2 file etc.
-pico_add_extra_outputs(hello_usb)
+    # create map/bin/hex/uf2 file etc.
+    pico_add_extra_outputs(hello_usb)
 
-# add url via pico_set_program_url
-example_auto_set_url(hello_usb)
+    # add url via pico_set_program_url
+    example_auto_set_url(hello_usb)
+elseif(PICO_ON_DEVICE)
+    message(WARNING "not building hello_usb because TinyUSB submodule is not initialized in the SDK")
+endif()