image-oci: add host layer type and delta-only copying

Add two enhancements to multi-layer OCI image support:

1. Delta-only copying for directories/files layers:
   - directories and files layers now only copy content that doesn't
     already exist in the bundle rootfs from earlier layers
   - Prevents duplication when a directories layer references paths
     that were already populated by a packages layer
   - Logs show "delta: N copied, M skipped" for visibility

2. New 'host' layer type for build machine content:
   - Copies files from the build machine filesystem (outside Yocto)
   - Format: name:host:source_path:dest_path
   - Multiple pairs: name:host:src1:dst1+src2:dst2
   - Emits warning at parse time about reproducibility impact
   - Fatal error if source path doesn't exist
   - Use case: deployment-specific config, certificates, keys that
     cannot be packaged in recipes

Example:
  OCI_LAYERS = "\
      base:packages:busybox \
      app:directories:/opt/myapp \
      certs:host:/etc/ssl/certs/ca.crt:/etc/ssl/certs/ca.crt \
  "

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

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