nanobsd: Source legacy.sh on demand instead of unconditionally

[senguptaangshuman17_gmail.com]

Summary

Previously nanobsd.sh unconditionally sourced legacy.sh at startup.
Instead, source it on demand through the legacy() plan function, and
fall back to it only when NANO_PLAN is unset, with a warning so
existing configs know to set NANO_PLAN explicitly.

[senguptaangshuman17_gmail.com]

Rebase stack on top of D59043 update

[jlduran]

This revision is just a placeholder for D54187?

[senguptaangshuman17_gmail.com]

In D59166#1357692, @jlduran wrote:

This revision is just a placeholder for D54187?

yes with just small modification in tools/tools/nanobsd/embedded/common where NANO_PLAN=embedded is defined

[jlduran]

In D59166#1357696, @senguptaangshuman17_gmail.com wrote:

In D59166#1357692, @jlduran wrote:

This revision is just a placeholder for D54187?

yes with just small modification in tools/tools/nanobsd/embedded/common where NANO_PLAN=embedded is defined

In that case, please explicitly add @imp as a reviewer or suggest changes in D54187.
I have to revisit is_defined() because it's been a while.