Enable release build and -Os optimization

Without -Os and with debug build -Og is enabled. This resulted in
unstable firmware flash times (30 vs 40 seconds) that seem to depend
on the minute details of the generated code layout.

software/platformio.ini

@@ -19,14 +19,15 @@ platform = platformio/espressif32@^6.7.0
 framework = arduino
 
 monitor_speed = 115200
-build_type = debug
+build_type = release
 
 extra_scripts = pre:pio_hooks.py
                 post:merge_firmware_hook.py
 
 board_build.partitions = default_16MB_coredump.csv
 
-build_src_flags = -Wall
+build_src_flags = -Os
+                  -Wall
                   -Wextra
                   -Wshadow=local
                   -Werror=return-type