Misc fixes to release tools.

- Remove vendor_dlkm and odm_dlkm from the default list of items that
need to be merged
- Allow a prebuilt system.img to be used when adding images to the
target files archive

Test: Built Pixel 3a and inspected the output
Change-Id: If5291fff559ed9517133f6b476d8dee2500c5b4c

tools/releasetools/merge_target_files.py

@@ -197,8 +197,6 @@ DEFAULT_VENDOR_ITEM_LIST = (
     'PREBUILT_IMAGES/*',
     'RADIO/*',
     'VENDOR/*',
-    'VENDOR_DLKM/*',
-    'ODM_DLKM/*',
 )
 
 # VENDOR_EXTRACT_SPECIAL_ITEM_LIST is a list of items to extract from the