initrd: set a default carrier timeout of 10 seconds in initrd

In initrd, a too short carrier timeout means that the machine will
possibly fail to boot. On the other hand, increasing the value doesn't
have side effects, except for a bit longer delay on some machines.

Increase the value to 10 seconds. Note that the default value is not
propagated to the real root.

!1239
59 jobs for bg/initrd-carrier-timeout in 37 minutes and 21 seconds (queued for 3 seconds)
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