Add a new function backtrace_str() that returns a condensed backtrace
string containing function names and line numbers separated by " <-".
For example: "func_a:123 <-func_b:456 <-func_c:789"
This is useful for logging and debugging where a compact representation
of the call stack is needed. The depth is controlled by the new
CONFIG_BACKTRACE_DEPTH option (default 3).
Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>
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