scripts: fix dumping execution backtrace

Dumping the backtrace has been broken since changeset #652e56d6d35a:
scripts: execute each steps in a subshell

We can spawn sub-sub-shells in some cases.

The way the fault handler works is to dump the backtrace, but to avoid
printing it once for every sub-shell (which could get quite confusing),
it simply exits when it detects that it is being run in a sub-shell,
leaving to the top-level shell the work to dump the backtrace.

Because each step is executed in its own sub-shell, the variable arrays
that contain the step name, the source file and line number, are lost
when exiting the per-step sub-shell.

Hence, the backtrace is currently limited to printing only the top-level
main procedure of the shell.

Fix this thus:
- when dumping the bckatraces for the steps & the functions, remember
it was dumped, and only dump it if it was not already dumped
- at the top-level shell, print the hints

Also, rename the top-level step label.

Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 4193d6e6a17430a177fa88c287879c2c35e319f3)

This is a somewhat ordered TODO list:

Recurring tasks:

- update versions for every tools...

- update and/or upgrade all samples

Non-recurring tasks:

- update newlib (for enhanced bare metal)

  - confirm existing implementation on targets other than AVR32

  - try to make it generic, will help for uClibc++

- multilib

- Cygwin target (needs newlib)

  - so we can build toolchains that generate code for Windows/Cygwin

- Cygwin host

  - so we can build toolchains that run natively under Windows/Cygwin

- mingw32 target

  - so we can build toolchains that generate code for Windows/Win32

- mingw32 host

  - so we can build toolchains that run natively under Windows/Win32

    (without requiring Cygwin)

- uClibc++

  - see newlib, above

- uClinux

  - check whether we can use newlib under uClinux (?)