Remove hw directory and replace it in GCS

[kchygoe]

• Remove hw/intel/de10_nano directory
• Place sample OS image/config files on a bucket, gs://blueoil-asia-northeast1
• Fix document to download them from the bucket

What this patch does to fix the issue.

HW directory includes binaries and makes repo size big.
These files should be place in a bucket.

Link to any relevant issues or pull requests.

#619

[iizukak]

OA
Nice PR~

[hadusam]

LGTM:+1:

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[kchygoe]

/ready

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