fs_config_generator: Add fs_config_(dirs|files)_(vendor|oem|odm)

Add build targets to generate the vendor, oem and odm slices of the
fs_config_dirs and fs_config_files targets for each of the respective
filesystems, and automatically trigger the individual and enabled
partition build objects based as dependents of fs_config_dirs and
fs_config_files.

Sensitive to the following board definition pairs:

BOARD_VENDORIMAGE_FILE_SYSTEM_TYPE  BOARD_USES_VENDORIMAGE
BOARD_OEMIMAGE_FILE_SYSTEM_TYPE     BOARD_USES_OEMIMAGE
BOARD_ODMIMAGE_FILE_SYSTEM_TYPE     BOARD_USES_ODMIMAGE

Which when defined and set means the images are generated for vendor,
oem or odm respectively.

Test: manually build w/ m 'fs_config_(dirs|files)_(vendor|oem|odm)'
      and check the expected products, or fails if not pertinent.
Test: build for product that produces split to system and vendor and
      subsequently boots with vendor products correctly configured.
Bug: 36071012
Change-Id: I44289a3dd50b7657367c826ad813c671a3de6afc