modules: rpi_pico: fix boot_stage2 generation when using Makefiles

RPi Pico second stage bootloader generation seems to fail when using
Makefiles with:

gmake[2]: *** No rule to make target 'bootloader/boot_stage2.S'...

Changing the ExternalProject_Add byproduct to the actual generated file
and not the .S seems to fix the problem.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>