pkgbase: Add incremental packages

This adds a new target update-packages which will create the new packages
compared to the last run.

This is how to use it:
At this point we cut a release
$ make buildworld ...
$ make buildkernel
$ make packages

    There is now a PKG_VERSION directory with latest link pointing to it
    Distribute the packages to server

$ something something that update the source tree
$ make buildworld ...
$ make buildkernel
$ make update-packages
You know have a PKG_VERSION directory in the REPODIR and latest link pointing to it.
In PKG_VERSION dir only the packages which differs from the latest run are
named PKG_VERSION, otherwise the old packages are there.

The process is :
Build the new packages in the PKG_VERSION directory
Compare the internal data with the PKG_VERSION_FROM version. The comparison is done
by checking the internal hash of the packages.
By default PKG_VERSION_FROM is set to what the latest link points to.
If the old and new version matches, we rm the new package and cp the old one.

Differential Revision:	https://reviews.freebsd.org/D25984

Makefile

@@ -162,8 +162,8 @@ TGTS=	all all-man buildenv buildenvvars buildkernel buildworld \
 	xdev-links native-xtools native-xtools-install stageworld stagekernel \
 	stage-packages stage-packages-kernel stage-packages-world \
 	create-packages-world create-packages-kernel create-packages \
-	packages installconfig real-packages sign-packages package-pkg \
-	print-dir test-system-compiler test-system-linker
+	update-packages packages installconfig real-packages real-update-packages \
+	sign-packages package-pkg print-dir test-system-compiler test-system-linker
 
 # These targets require a TARGET and TARGET_ARCH be defined.
 XTGTS=	native-xtools native-xtools-install xdev xdev-build xdev-install \