scripts: add support of tfm subpartitions

Add the support of the tfm subpartitions and simplify the code using an
AI agent.

Assisted-by: GitHub Copilot:Claude Haiku 4.5
Signed-off-by: Riadh Ghaddab <riadh.ghaddab@nordicsemi.no>
(cherry picked from commit 1ff45fe)

Author: rghaddab