rp2/Makefile: Regenerate cmake if generated Makefile missing.

`CMakeCache.txt` can exist even if cmake failed. Use `Makefile` instead.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/rp2/Makefile

@@ -23,7 +23,7 @@ CMAKE_ARGS += -DCMAKE_BUILD_TYPE=Debug
 endif
 
 all:
-	[ -e $(BUILD)/CMakeCache.txt ] || cmake -S . -B $(BUILD) -DPICO_BUILD_DOCS=0 ${CMAKE_ARGS}
+	[ -e $(BUILD)/Makefile ] || cmake -S . -B $(BUILD) -DPICO_BUILD_DOCS=0 ${CMAKE_ARGS}
 	$(MAKE) $(MAKESILENT) -C $(BUILD)
 
 clean: