Fixed PREMIRRORS in the local.conf template

"PREMIRRORS =+" does not work when own-mirrors are specified
in local.conf. "PREMIRRORS_prepend =" should be used instead

JIRA: MEIBPADIT-737

Signed-off-by: Mikhail Durnev <Mikhail_Durnev@mentor.com>
Merge pull request #85 from mdurnev/mdurnev/premirrors

Fixed PREMIRRORS in the local.conf template