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)

#!@@CT_bash@@

# What we need:

#  - the .config file

# Parse the tools' paths configuration

. "${CT_LIB_DIR}/paths.mk"

# We'll need the stdout later, save it

exec 7>&1

# Parse the common functions

. "${CT_LIB_DIR}/scripts/functions"

# Don't care about any log file

exec >/dev/null

rm -f "${tmp_log_file}"

# Parse the configuration file

. .config.2

# Parse architecture and kernel specific functions

. "${CT_LIB_DIR}/scripts/build/arch/${CT_ARCH}.sh"

. "${CT_LIB_DIR}/scripts/build/kernel/${CT_KERNEL}.sh"

# Build CT_TARGET

CT_DoBuildTargetTuple

# All this for this single echo... :-( Sigh, I'll have to re-arrange things...

echo "${CT_TARGET}" >&7