optee-utee: make panic_handler optional in no-std by adding "no_panic_handler" feature flag

[ivila]

This PR is going to provide a feature flag no_panic_handler, which allow to disable the default panic_handler optee_utee provided, and let the TA developer develop their own one.
Relative issue is here: #145

[DemesneGH]

@ivila Thanks for your contribution! Since the main branch is the current developing branch, could you rebase on the main branch and recreate a PR? thanks!

[DemesneGH]

Sorry for a pending PR for main, I have merged that so it's good to be rebased on now

[ivila]

Sorry for a pending PR for main, I have merged that so it's good to be rebased on now

You can do that first, I am kind of busy now, and I should recreate a valid PR about 15:00 today.

[b49020]

Care to update commit header and description as follows?

no-std: optee-utee: make panic_handler optional

Make panic_handler optional for no-std TAs by adding "no_panic_handler"
feature flag. It allows no-std TAs to have a custom panic handler.

[ivila]

PR close due to using the wrong branch (should use main branch instead of no-std)
The recreate one is #147