Image import keeps the shared BPRT link and seeds BPRT on a fresh box

An MCL import merged Deploy/Applications with rsync -a, which replaced each
image's BPRT symlink with a plain copy of the vendor folder. That copy has
no site overlay, so standard/engineer enrollment lost
Register-BulkEnrollTask-Standard.ps1 (specialize Order 39).

Now Deploy/Applications is imported first and without BPRT:
- where _shared/Applications/BPRT exists it is left alone (the BPRT page
  keeps it current) and the image link is put back;
- on a box with none yet, the imported BPRT seeds it: vendor
  Select-PCConfig.ps1 kept as -vendor, our overlay added, BOM fix applied;
- the import refuses before anything moves when that folder would not work
  (vendor scripts missing, or no overlay copy to add).

The two Register scripts join Select-PCConfig.ps1 in the repo. The playbook
already syncs shopfloor-setup to the enrollment share, so
config.BPRT_OVERLAY_DIR finds all three there; overlay_sources now falls
back to it for every overlay file, not only the shim.

webapp/config.py

@@ -15,6 +15,11 @@ ENROLLMENT_SHARE = os.environ.get("ENROLLMENT_SHARE", "/srv/samba/enrollment")
 ENROLLMENT_PPKG_DIR = os.environ.get(
     "ENROLLMENT_PPKG_DIR", os.path.join(ENROLLMENT_SHARE, "ppkgs")
 )
+# Our BPRT overlay scripts (repo playbook/shopfloor-setup/BPRT/), which the
+# playbook syncs here with the rest of shopfloor-setup.
+BPRT_OVERLAY_DIR = os.environ.get(
+    "BPRT_OVERLAY_DIR", os.path.join(ENROLLMENT_SHARE, "shopfloor-setup", "BPRT")
+)
 UPLOAD_DIR = os.environ.get("UPLOAD_DIR", "/home/pxe/image-upload")
 SHARED_DIR = os.path.join(SAMBA_SHARE, "_shared")
 WEB_ROOT = os.environ.get("WEB_ROOT", "/var/www/html")