ref-manual: variables: add TARGET_DBGSRC_DIR

New variable in 5.0.

(From yocto-docs rev: 047780931ec8156d77ec10f3f051bd98b83714f8)

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -8813,6 +8813,10 @@ system and gives an overview of their function and contents.
       value so that executables built using the SDK also have the flags
       applied.
 
+   :term:`TARGET_DBGSRC_DIR`
+      Specifies the target path to debug source files. The default is
+      ``/usr/src/debug/${PN}/${PV}``.
+
    :term:`TARGET_FPU`
       Specifies the method for handling FPU code. For FPU-less targets,
       which include most ARM CPUs, the variable must be set to "soft". If