meta-ti/all: Handle S when building a subdir of the git repo

If a recipe is fetching an entire git repo and then just building a
single dir, then we need to update S to lock into the sub directory.
Easiest fix is to simply append the subdir to whatever the system wants
to set S to by default.

Signed-off-by: Ryan Eatmon <reatmon@ti.com>

meta-ti-extras/recipes-devtools/gdbc6x/gdbserverproxy-module-drv_git.bb

@@ -9,7 +9,7 @@ SRC_URI:append = "\
     file://0002-Support-Linux-kernels-v6.6.patch;patchdir=../.. \
 "
 
-S = "${WORKDIR}/git/kernel_module/gdbproxy-mod"
+S:append = "/kernel_module/gdbproxy-mod"
 
 inherit module