Generate init.environ.rc by populating BOOTCLASSPATH

- BOOTCLASSPATH now is derived from PRODUCT_BOOT_JARS, which is a product
  configuration variable set up by the core build system.
- Moved files from the legacy ALL_PREBUILT to PRODUCT_COPY_FILES in
  build/target/product/embedded.mk.

Bug: 9990214
Change-Id: I98bac36c1ca8c779dda572a0a5e0a22b7e4c4a7a