imx6ull-uart: add wait for root before create_dev()

[niewim19]

JIRA: PP-7

Description

Add wait for root before create_dev() in imx6ull uart.

Motivation and Context

When working on imx6ull devkit no /dev/ appeared without this change, thus making working with devices impossible.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

How Has This Been Tested?

• [ ] Already covered by automatic testing.
• [ ] New test added: (add PR link here).
• [x] Tested by hand on: imx6ull

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing linter checks and tests passed.
• [x] My changes generate no new compilation warnings for any of the targets.

Special treatment

• [ ] This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• [ ] I will merge this PR by myself when appropriate.

[agkaminski]

@ziemleszcz Won't this PR break DTR project? From what I remember rootfs might wait for console on the other hand (this would result in deadlock). That was the whole point of devfs

[ziemleszcz]

Looks like it's not blocking. I don't see imx6ull-flash waiting for console.

[nalajcie]

Looking at the code - it won't break the DTR project, but it's worth to check it anyway (can You build and test it @niewim19?)

IMHO this should be a part of create_dev if it's needed for device creation... aaaand on DTR it's not needed due to devfs mount point being used.

[badochov]

Can this PR be closed as no longer needed now?

[nalajcie]

I see the armv7a7-imx6ull-evk project uses devfs approach now so there is no need to wait for rootfs. Closing this PR.