support mkfs for userspace convertor

[liulanzheng]

What this PR does / why we need it:

Add an flag --mkfs for userspace convertor, which make ext filesystem in the bottom layer, base layer is not required under this config.
mkfs will be the default config in the feature.
Be carefull, it may be conflict with layer deduplication if reused layers were built without mkfs.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[liulanzheng]

@WaberZhuang cc

[WaberZhuang]

LGTM