backtrace: Add backtrace_str() for condensed backtrace output

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>

lib/Kconfig

@@ -36,6 +36,15 @@ config BACKTRACE
 	  stack. This is currently only available on sandbox. The backtrace
 	  command can be used to print the backtrace.
 
+config BACKTRACE_SUMMARY_FRAMES
+	int "Number of frames in condensed backtrace"
+	depends on BACKTRACE
+	default 3
+	help
+	  Number of stack frames to include in the condensed backtrace
+	  string returned by backtrace_str(). This affects BSS usage
+	  since space must be allocated for the string.
+
 config LIB_FORMAT_SIZE
 	bool
 	default y