release.sh: add chroot cleanup routine

[ngie]

Summary

The chroot_cleanup routine handles any cleanup needed post-chroot_setup,
etc. This consists of purely tearing down ${CHROOTDIR}/dev today, but
might involve additional steps, as needed for custom functions. This
allows end-users to override the various chroot functions without having
to modify code in main() or replicate the unmount procedure in an
equivalent routine setup with trap(1).

This change modifies the /dev unmount process to use umount -f instead
of umount. The latter can result in failures if resources are still
mounted or are running post-build, whereas the former will clean up any
resources still in use by processes running in the chroot at time of
build. Moreover, the chroot_cleanup routine is now called when the
script is killed with SIGINT and SIGTERM, as well as at EXIT, better
ensuring that the script's resources are cleaned up in relatively common
scenarios that can be detected/handled.

While here, document override functions to aid others in overriding them.

MFC after: 1 week

Test Plan

Executing the script via: sh release/release.sh -c release/arm64/NVIDIA.conf.

[ngie]

@cperciva : ping?

[cperciva]

Personally I would put "move umount /dev into a new function" and "add comments" into different commits, but since you've already done it this way there's no point splitting them apart.

[ngie]

In D55450#1281179, @cperciva wrote:

Personally I would put "move umount /dev into a new function" and "add comments" into different commits, but since you've already done it this way there's no point splitting them apart.

I'm totally fine doing that :). Adding appropriate documentation in a separate commit is the right thing to do.