Some mmc controllers have no card detect pin, so the only
way to detect card presence is to issue the ACMD41 which will
fail after a pretty long timeout.
To avoid mmconline() blocking, we only try to initialize the
card synchronous once, and then retry in a background process,
while returning immediately from mmconline() while the retry
is in progress.
This speeds up network boot times significantly on a raspi
without a sdcard inserted.
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