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)

 From 323453d85ed29ccba7a81d884479188869cd64b7 Mon Sep 17 00:00:00 2001

 From: Henning Heinold <heinold@inf.fu-berlin.de>

 Date: Sat, 23 Jan 2010 21:29:42 -0800

 Subject: [PATCH 13/15] Makefile.in: Make install_dev depend on install_runtime.

 * Helps in parallel build.

 Signed-off-by: Henning Heinold <heinold@inf.fu-berlin.de>

 Signed-off-by: Khem Raj <raj.khem@gmail.com>

 Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

 ---

  Makefile.in |    2 +-

  1 files changed, 1 insertions(+), 1 deletions(-)

 diff --git a/Makefile.in b/Makefile.in

 index ec4c28e..a889838 100644

 --- a/Makefile.in

 +++ b/Makefile.in

 @@ -370,7 +370,7 @@ ifneq ($(UCLIBC_SUPPORT_AI_ADDRCONFIG),y)

  endif

  # Installs development library links.

 -install_dev: install_headers all

 +install_dev: install_headers install_runtime

  	$(INSTALL) -d $(PREFIX)$(DEVEL_PREFIX)lib

  	-$(INSTALL) -m 644 lib/*.[ao] $(PREFIX)$(DEVEL_PREFIX)lib/

  ifeq ($(HAVE_SHARED),y)

 -- 

 1.6.6.1