Misc cleanup (#2569)

* miscellaneous cleanup:

* cleanup some #ifdefs which were slightly hacky when RP2350 was added; use HAS_ flags in preference to PICO_RP2040/RP2350
* make some dependencies more explicit - i.e. compile if the user doesn't want to include certain libraries
* cleanup some directory A -> directory B relative path names in CMakeLists.txt to be SDK root -> directory B

test/pico_time_test/pico_time_test.c

@@ -41,8 +41,8 @@ static struct timeout {
     alarm_id_t alarm_id;
     absolute_time_t target;
     absolute_time_t fired_at;
-    uint pool;
     uint fired_count;
+    uint8_t pool;
     bool cancelled;
     bool not_cancelled; // tried to cancel but it was done
 } timeouts[NUM_TIMEOUTS];