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)

 # Kernel options

 menu "Operating System"

 # Config option used throughout the config and code to determine wether

 # we have a kernel or not (there might be different bare metal stuff)...

 config BARE_METAL

     bool

 config MINGW32

     bool

 # Each target OS (aka kernel) that support shared libraries can select

 # this, so the user can decide whether or not to build a shared library

 # enabled toolchain

 config KERNEL_SUPPORTS_SHARED_LIBS

     bool

 config KERNEL

     string

 config KERNEL_VERSION

     string

 source "config.gen/kernel.in"

 comment "Common kernel options"

 config SHARED_LIBS

     bool

     prompt "Build shared libraries"

     depends on KERNEL_SUPPORTS_SHARED_LIBS

     default y

     help

       Say 'y' here, unless you don't want shared libraries.

       You might not want shared libraries if you're building for a target that

       don't support it (maybe some nommu targets, for example, or bare metal).

 source "config.gen/kernel.in.2"

 endmenu