init: poll in first stage mount if required devices are not found

First stage mount in init currently attempts to regenerate uevents for
specific devices to create the corresponding dev nodes.  However, this
is racy as first stage mount happens early in the boot process and
it's possible that some of these devices have not yet been created by
the kernel.

To fix this issue, init will poll on the uevent socket for up to 10
seconds waiting for the kernel to create the required device.  It will
return false and panic if this 10 second timeout passes.

Note that the same uevent socket is used in the uevent regeneration
and the polling code, so there is no race if the device is created
after the uevent regeneration and before polling starts; the first
poll will pick up the device.

Bug: 62681642
Bug: 62682821
Test: Boot bullhead
Test: Boot sailfish
Test: Boot hikey + hotplug/unplug sdcard
Merged-In: I4a6ff043eb7115b729ca4954ebc6c9e000132993
Change-Id: I4a6ff043eb7115b729ca4954ebc6c9e000132993