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>

 commit 608e138586993d3c59c65c2958d9a98cf0d4d1eb

 Author: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

 Date:   Wed Jun 8 21:47:42 2011 +0200

     bump version to 0.9.32.1-git

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

 diff --git a/Rules.mak b/Rules.mak

 index fe06c24..a00deea 100644

 --- a/Rules.mak

 +++ b/Rules.mak

 @@ -106,7 +106,7 @@ export RUNTIME_PREFIX DEVEL_PREFIX KERNEL_HEADERS MULTILIB_DIR

  MAJOR_VERSION := 0

  MINOR_VERSION := 9

  SUBLEVEL      := 32

 -EXTRAVERSION  :=

 +EXTRAVERSION  :=.1-git

  VERSION       := $(MAJOR_VERSION).$(MINOR_VERSION).$(SUBLEVEL)

  ABI_VERSION   := $(MAJOR_VERSION)

  ifneq ($(EXTRAVERSION),)