backtrace: Add a command

Add a new 'backtrace' command which prints the current call stack, which
is useful for debugging. The command is enabled by CONFIG_CMD_BACKTRACE

Add docs and a test.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

doc/usage/cmd/backtrace.rst

@@ -0,0 +1,51 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+.. index::
+   single: backtrace (command)
+
+backtrace command
+=================
+
+Synopsis
+--------
+
+::
+
+    backtrace
+
+Description
+-----------
+
+The *backtrace* command prints a backtrace of the current call stack. This can
+be useful for debugging to see how a particular code path was reached.
+
+The output shows each stack frame with the function name, source file, and line
+number (when debug information is available). This includes static functions.
+
+Example
+-------
+
+::
+
+    => backtrace
+    backtrace: 14 addresses
+      backtrace_show() at /home/user/u-boot/lib/backtrace.c:17
+      do_backtrace() at /home/user/u-boot/cmd/backtrace.c:18
+      cmd_process() at /home/user/u-boot/common/command.c:637
+      run_list_real() at /home/user/u-boot/common/cli_hush.c:1868
+      parse_stream_outer() at /home/user/u-boot/common/cli_hush.c:3207
+      parse_string_outer() at /home/user/u-boot/common/cli_hush.c:3257
+      run_command_list() at /home/user/u-boot/common/cli.c:168
+      sandbox_main_loop_init() at /home/user/u-boot/arch/sandbox/cpu/start.c:153
+      board_init_r() at /home/user/u-boot/common/board_r.c:774
+      ...
+
+Configuration
+-------------
+
+The backtrace command is enabled by CONFIG_CMD_BACKTRACE which depends on
+CONFIG_BACKTRACE. Currently this is only available on sandbox.
+
+The sandbox implementation uses libbacktrace (bundled with GCC) to provide
+detailed symbol information including function names, source files, and line
+numbers.

doc/usage/index.rst

@@ -31,6 +31,7 @@ Shell commands
    cmd/addrmap
    cmd/armffa
    cmd/askenv
+   cmd/backtrace
    cmd/base
    cmd/bdinfo
    cmd/bind