Move function to enable field mode to common code

The only different between ubi and static layout for setting field mode
was masking the usr-local.mount service file. Now that the service file
is in the common code, it makes sense to move the function to the common
code.

Tested:
- Witherspoon: Verified setting field mode disabled the /usr/local mount
as it currently does.
- Romulus: Verified that after setting the field mode, the usr-local.mount
service file could not be started because it's masked.

Change-Id: I8afb755366053c38f72d682e874308c42e232d98
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
4 files changed