[fpv/flash] Fix flash conpile error
[fpv/flash] Fix flash conpile error

This PR fixed flash compile error by overriding the `design_level` to
`top` in order to import the correct files.

Signed-off-by: Cindy Chen <399ad771a111eac8923348b1e2af093034e5d4f1@opentitan.org>

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