PR#45836 lang/openjd7: Repack DragonFly bootstraps

OpenJDK7 wouldn't build on DragonFly for non-root users due to a conflict
with the bootstrap/LICENSE file.  Both the -bin-common and the
-bin-dragonfly bootstraps contained the same file, both with 444 file
permissions.  As a result, the extraction phase fails for non-root pbulk
builds and other under-privileged users.

The DragonFly bootstraps were repacked to exclude the duplicate
bootstrap/LICENSE file, and the bootstrap.mk file updated accordingly.
The new bootstraps are packed with xz, resulting in a tarball 6MB
smaller for i386.

Other changes while we're here:
1) Add LICENSE=gnu-gpl-v2
2) USE_TOOLS+= patch (pkglint complained)
3) Fix ONLY_FOR_PLATFORM triplet for DragonFly (pkglint complained)

(marino)