verification/rvgen: Organise Kconfig entries for nested monitors

The current behaviour of rvgen when running with the -a option is to
append the necessary lines at the end of the configuration for Kconfig,
Makefile and tracepoints.
This is not always the desired behaviour in case of nested monitors:
while tracepoints are not affected by nesting and the Makefile's only
requirement is that the parent monitor is built before its children, in
the Kconfig it is better to have children defined right after their
parent, otherwise the result has wrong indentation:

[*]   foo_parent monitor
[*]     foo_child1 monitor
[*]     foo_child2 monitor
[*]   bar_parent monitor
[*]     bar_child1 monitor
[*]     bar_child2 monitor
[*]   foo_child3 monitor
[*]   foo_child4 monitor

Adapt rvgen to look for a different marker for nested monitors in the
Kconfig file and append the line right after the last sibling, instead
of the last monitor.
Also add the marker when creating a new parent monitor.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Tomas Glozar <tglozar@redhat.com>
Cc: Juri Lelli <jlelli@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Link: https://lore.kernel.org/20250723161240.194860-5-gmonaco@redhat.com


Reviewed-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>