release: allow using DESTDIR with ftp target

[royger]

Summary

Allow specifying a destination directory for the install sets
generated by the release ftp target.

No functional change expected unless DESTDIR is specified when calling
the ftp target.

Sponsored by: Citrix Systems R&D

[gjb]

This breaks the default behavior when .OBJDIR is not set.

This is a more correct fix:

release_Makefile.txt542 BDownload

[gjb]

Output from your change without the added ${.OBJDIR}:

root@releng1:/usr/src/release # make ftp
rm -rf /ftp
mkdir -p /ftp
cp *.txz MANIFEST /ftp
root@releng1:/usr/src/release # ls /ftp/
MANIFEST        base-dbg.txz    base.txz        kernel-dbg.txz  kernel.txz      lib32-dbg.txz   lib32.txz       ports.txz       src.txz         tests.txz