Steps to reproduce:
1. boot with 'root=/path/to/mmc_card' and without 'rootwait'

Expected results:
the system boots

Actual results:
---8<---
VFS: Cannot open root device "mmcblk0p1" or unknown-block(0,0): error -6
Please append a correct "root=" boot option; here are the available partitions:
mmc0: host does not support reading read-only switch. assuming write-enable.
1f00             256 mtdblock0  (driver?)
1f01             256 mtdblock1  (driver?)
1f02            2560 mtdblock2 mmc0: new SDHC card at address b368
 (driver?)
1f03           29696 mtdblock3  (driver?)
1f04           16384 mtdblock4 mmcblk0: mmc0:b368 USD   3.72 GiB
 (driver?)
 mmcblk0: p1
b300         3910656 mmcblk0  driver: mmcblk
  b301         3906560 mmcblk0p1 00000000-01
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
---8<---

This happens because prepare_namespace() assumes that all devices are ready when all probe() finish, but mmc_start_host() returns asynchronously before the card is properly initialized, thus causing a hard to trace race condition.

The patch was committed which makes mmc_start_host() synchronous, but it was reverted without another solution, because it increases boot time on some systems.

As a result, 'root_wait' is required to ensure boot at the moment.

A possible solution is to make card a separate device (now only the host is a device). In this case, the probing could be done asynchronously without breaking the assumption in prepare_namespace().