flash.sh: Add warning to remind user to not interfere with flashrom operations that will follow

[tlaurion]

Add a warning prior of calling flashrom reminding end user to not poweroff and wait while flashing operation finishes.

Message prior of version dependent flashrom output will now be :

WARNING: Do not power off computer. Updating firmware, this will take a few minutes...

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Reasoning, as per https://github.com/linuxboot/heads/pull/1753#issuecomment-2307255469:

So on master with flashrom 1.3:

• tested (DEBUG ON) https://output.circle-artifacts.com/output/job/78a45204-7736-4f87-bf5d-922b48469782/artifacts/0/build/x86/x230-hotp-maximized/heads-x230-hotp-maximized-v0.2.0-2282-g1e03e8c.zip
• Scary, but ok:

Under this PR containing flashrom 1.4:

• tested (DEBUG ON) https://output.circle-artifacts.com/output/job/31dce9e2-7216-4d90-9528-efd3d0ff6668/artifacts/0/build/x86/x230-hotp-maximized/heads-x230-hotp-maximized-v0.2.0-2290-g35b43b0.zip
• Scary but not so ok: