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)

# Linux kernel options

if ! KERNEL_LINUX_USE_CUSTOM_HEADERS

choice

    bool

    prompt "Kernel verbosity:"

    default KERNEL_LINUX_VERBOSITY_0

config KERNEL_LINUX_VERBOSITY_0

    bool

    prompt "Simplified"

    help

      Print simplified command lines.

config KERNEL_LINUX_VERBOSITY_1

    bool

    prompt "Full commands"

    help

      Print full command lines.

config KERNEL_LINUX_VERBOSITY_2

    bool

    prompt "Exec reasons"

    help

      Print the reasons why a make target is rebuild.

endchoice

config KERNEL_LINUX_VERBOSE_LEVEL

    int

    default 0 if KERNEL_LINUX_VERBOSITY_0

    default 1 if KERNEL_LINUX_VERBOSITY_1

    default 2 if KERNEL_LINUX_VERBOSITY_2

config KERNEL_LINUX_INSTALL_CHECK

    bool

    prompt "Check installed headers"

    default y

    help

      If you are in doubt that installed headers are buggy, say 'Y'

      here to have an extra check passed onto the headers.

endif # ! KERNEL_LINUX_USE_CUSTOM_HEADERS