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)

# newlib second-part options

config LIBC_NEWLIB_IO_C99FMT

    bool

    prompt "Enable IOs on C99 formats"

    help

      Enable support for IOs on C99 formats.

config LIBC_NEWLIB_IO_LL

    bool

    prompt "Enable IOs on long long"

    help

      Enable support for IOs on long long integers.

config LIBC_NEWLIB_IO_FLOAT

    bool

    prompt "Enable IOs on floats and doubles"

    help

      Enable support for IOs on floating point

      values (float and double).

config LIBC_NEWLIB_IO_LDBL

    bool

    prompt "Enable IOs on long doubles"

    depends on LIBC_NEWLIB_IO_FLOAT

    help

      Enable support for IOs on long doubles.

config LIBC_NEWLIB_DISABLE_SUPPLIED_SYSCALLS

    bool

    prompt "Disable the syscalls supplied with newlib"

    help

      Disable the syscalls that come with newlib. You

	  will have to implement your own _sbrk, _read,

	  _write... If you plan to port newlib to a new

	  platform/board, say Yes.