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)

# mingw32 config options

## depends on EXPERIMENTAL && ( ARCH_x86 && ARCH_32 )

##

## select MINGW32

##

## help Build a toolchain targeting systems running Windows as host

choice

    bool

    prompt "Windows api version"

# Don't remove next line

# CT_INSERT_VERSION_BELOW

config W32API_V_3_14

    bool

    prompt "3.14"

config W32API_V_select

    bool

    prompt "Other version"

endchoice

config W32API_VERSION

    string

    prompt "W32 api version" if W32API_V_select

# Don't remove next line

# CT_INSERT_VERSION_STRING_BELOW

    default "3.14" if W32API_V_3_14

    help

      Enter the version number of the windows api files to use