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)

# Menu for the native gdbserver

config GDB_GDBSERVER

    bool

    prompt "gdbserver"

    depends on ! BARE_METAL

    help

      Build and install a gdbserver for the target, to run on the target.

if GDB_GDBSERVER

config GDB_GDBSERVER_STATIC

    bool

    prompt "Build a static gdbserver"

    default y

    help

      In case you have trouble with dynamic loading of shared libraries,

      you will find that a static gdbserver comes in handy.

endif # GDB_GDBSERVER

if BARE_METAL

comment "In bare-metal, you'll need to   "

comment "provide your own gdbserver stub."

endif # BARE_METAL