Permit unsafe usb boot of unsigned ISO (Ventoy-like)

[tlaurion]

fixes

• Ventoy'ish boot of unsigned iso Heads cant boot from USB-drive with Ventoy #1320 (ExFat: supported by Heads since f6eed42)
• Not block user to boot unsigned ISO unsafe usb boot of iso files without integrity verification #1886

Changes

• /media is mounted as usual
• /media is searched without limit for files matching name "*.iso"
• same code path for detached signed boot images.
• when sig/asc not found, warn prior of booting that unsigned iso image

Screenshot (old ISOs with detached signature removed for sake of these tests: irrelevant):

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Todo:

• Boot from unsigned USB ISO: Change UX to propose to show iso hash for user first time validation + detach sign hashfile #1438
• cancelling anything on that boot path calls 'die' (which goes up to media-scan, landing into the recovery shell for now. PR welcome).

[tlaurion]

Satifying to close the following issues?

• Heads cant boot from USB-drive with Ventoy #1320: @natterangell @fhvyhjriur: satifying?
• unsafe usb boot of iso files without integrity verification #1886: @arhabd ?

Please thumb up/down+comment?

[tlaurion]

@JonathonHall-Purism we have two thumbs up at #1984 (comment) where having reviews is difficult. I think this PR is ready to merge unless someones oppose.