Update releasetools for new init_boot.img

This new init_boot.img contains the ramdisk that used to reside in the
boot.img file.

Test: set BOARD_PREBUILT_INIT_BOOT_IMAGE to an external init_boot.img
      - Check that "m" pulls in the init_boot.img to
      out/target/product/vsoc_x86_64/
      - Check that "m dist" adds the init_boot.img to
      aosp_cf_x86_64_phone-img-eng.devinmoore.zip
Test: atest --host releasetools_test
Bug: 203698939
Change-Id: If7ef2cf093e5e525529c7c44333c0f40f6ba0764