emulators/linux_base-c6: fix build/packaging as unprivileged user.

[xmj]

Summary

As Matthias Andree reported in 193848,
due to permissions CentOS 6.5 cannot currently be built / installed as
non-privileged user.

Fix this by using pre-extract to extract the filesystem and setup RPMs,
and subsequently fix flawed permissions.

emulators/linux_base-c6: fix build/packaging as unprivileged user.

- Allow build/packaging as unprivileged user.

PR:	193848
(Diffrev: placeholder)
Submitted by:	mandree
Approved by:	swills (mentor)

Test Plan

http://xmj.me/freebsd/buildlogs/linux_base-c6-6.5_2.log

[swills]

Approved

[xmj]

Committed revision 371573.