Support optional factory-fit non-removable memory devices

Some boards feature non-removable memory devices with a no-fit option that
can only detected by seeing whether the device responds or not once we
start trying to talk to it. To support these, defer drive allocation until
the UnitAttached service call. This is already designed to dynamically
add extra drives in case a shared MMC bus is found, so the only difference
is that the initial number of drives is 0, not 1. Note that there is still
a minimum of 1 drive per slot for removable slots, since a removable slot
may be empty at module initialisation time, and there's no way to
electrically distinguish an empty socket from a no-fit socket.
13 jobs for !3 with SDIO in 29 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Static Analysis
passed #62466
cross
copyright

00:00:00

passed #62467
cross
cppcheck

00:00:10

passed #62457
cross
gitattributes

00:00:01

passed #62458
cross
gitignore

00:00:01

passed #62461
cross
head_log

00:00:00

passed #62464
cross
head_whitesp

00:00:00

passed #62459
cross
license

00:00:00

passed #62463
cross
makefile

00:00:00

passed #62462
cross
merge_log

00:00:01

passed #62465
cross
merge_whitesp

00:00:01

passed #62460
cross
versionnum

00:00:00

 
  Softload
passed #62468
cross
softload

00:00:07

 
  Cleanup
passed #62469
cross
cleanup

00:00:01