[ticket/12771] Bug in profilefield_base_migration when used in ext

The base migration class should create lang entry for the new CPF
but it is supposed to strip it of phpbb_ prefix. As extensions
CPFs do not use phpbb_ prefix it will not create a lang entry.
This is as simple solution as it can be done.

PHPBB3-12771
[ticket/12771] Change var name

Change var name

PHPBB3-12771
[ticket/12771] Shanging substr to strpos

Logical change ...

PHPBB3-12771
Merge pull request #2658 from lucifer4o/ticket/12771

[ticket/12771] Bug in profilefield_base_migration when used in ext

* lucifer4o/ticket/12771:
  [ticket/12771] Shanging substr to strpos
  [ticket/12771] Change var name
  [ticket/12771] Bug in profilefield_base_migration when used in ext