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)

 # Template file for a debug utility

 # Put your download code here

 do_debug_foobar_get() {

     # For example:

     # CT_GetFile "foobar-${CT_FOOBAR_VERSION}" http://foobar.com/releases/

     :

 }

 # Put your extract code here

 do_debug_foobar_extract() {

     # For example:

     # CT_Extract "foobar-${CT_FOOBAR_VERSION}"

     # CT_Patch "foobar" "${CT_FOOBAR_VERSION}"

     :

 }

 # Put your build code here

 do_debug_foobar_build() {

     # For example:

     # mkdir -p "${CT_BUILD_DIR}/build-foobar"

     # CT_Pushd "${CT_BUILD_DIR}/build-foobar"

     # CT_DoExecLog CFG                                        \

     # "${CT_SRC_DIR}/foobar-${CT_FOOBAR_VERSION}/configure"   \

     #     --build=${CT_BUILD}                                 \

     #     --host=${CT_TARGET}                                 \

     #     --prefix=/usr                                       \

     #     --foobar-options

     # CT_DoExecLog ALL make

     # CT_DoExecLog ALL make DESTDIR="${CT_SYSROOT_DIR}" install

     # CT_Popd

     :

 }