automount: wait for an expected external card before using internal

[maxengel]

What

On a device with two microSD cards, automount's find_games scans for the external games card once. When the card has not enumerated yet at that moment (a cold boot, a card slow to come up), the scan finds nothing and the script binds the internal card over /storage/roms. Anything written to /storage/roms during that boot — game saves, a restore — lands on the internal card, where it lingers and later reads as that device's newest data. Observed on an Anbernic RG SP: 101 stale save files on the internal card with both cards inserted the whole time.

Fix

find_games retries the scan when the device expects an external card — system.merged.device is external (it has mounted one before) or system.gamesdevice is pinned. It waits only while a candidate block device is physically present but not yet readable (a non-internal disk over ~8 GB under /sys/block), so:

Situation	Added to boot
card present and ready (the normal boot)	0 s
card present, filesystem a moment slow	its own lateness
card removed	2 s, then internal
one-card device	0 s
card present but never readable	10 s, then internal

A one-card device has neither signal and never waits. The helper's sed expressions are single-quoted: a double-quoted "s#p[0-9]*$##" had bash expand $#.

Testing

• The real find_games body run under mocked block devices for the seven situations above (card present, card late, card removed, one-card, unset setting, pinned device with a late card, present-but-unreadable): each falls where the table says.
• A GENERIC_X64 build and two Anbernic H700 handhelds: the RG SP (two cards) binds its external card at boot with no wait and writes nothing to the internal one; the RG35XX SP (one card) assumes internal without waiting, as before.

No user-facing option or documentation changes.