boot/nxboot: fix Clang warnings for format and va_start

[neilberkman]

Summary

Fix two Clang -Werror failures when building nxboot with CONFIG_ARM_TOOLCHAIN_CLANG:

• loader/flash.c: use PRIdOFF instead of %ld for off_t arguments in syslog calls, fixing -Wformat when off_t is not long.
• nxboot_main.c / nxboot.h: change nxboot_progress parameter from enum progress_type_e to int to avoid undefined behavior from va_start on a parameter that undergoes default argument promotion (-Wvarargs).

Impact

Fixes build failures for any board config that enables nxboot with Clang (e.g. nucleo-h743zi/nxboot-loader, nucleo-h743zi/nxboot-app in apache/nuttx#18509).

Testing

Build: nucleo-h743zi/nxboot-loader and nucleo-h743zi/nxboot-app with CONFIG_ARM_TOOLCHAIN_CLANG.

[linguini1]

Just noting the failure can be seen: https://github.com/apache/nuttx/actions/runs/22828785374/job/66215539259?pr=18509

[cederom]

@michallenc could you please take a look as the code author? :-)

[michallenc]

LGTM. The canceled checks in CI are correct?

[cederom]

Thank you @michallenc :-) GH CI is problematic recently. CI restarted :-)

[cederom]

Thank you @neilberkman :-) Lets just make sure CI pass fine :-)

[neilberkman]

Thank you @neilberkman :-) Lets just make sure CI pass fine :-)

Thanks! Unfortunately, looks like the rerun failed as well. This does all appear to be due to infrastructure issues, unrelated to the PR itself.

[cederom]

Yup GH seems to be doing some upgrades recently and lots of CI fails due to different reasons :-P