Force PREFIX to LINUXBASE.

[xmj]

Summary

As reported by @bapt, installing linux packages can conflict with FreeBSD
versions of the same software *if* PREFIX is overridden (and, e.g., set to
/usr/local) in /etc/make.conf.

We already allow the user to override LINUXBASE, so they should be able to use
this as (forced) prefix if a port uses the linux prefix settings.

Test Plan

• set PREFIX to /var/local
• set LINUXBASE to /compat/linux32
• poudriere testport -I [..] -o emulators/linux_base-c6
• log in to jail, notice bsd local stuff in /var/local, linux base stuff in /compat/linux32.

[koobs]

This LGTM, but also needs portmgr approval to proceed

[swills]

Approved

[xmj]

Committed revision 374414. (Mk/bsd.port.mk change)
Committed revision 374415 (emulators/linux_base-{c6,f10} changes)