Hacky workaround to run do_build.sh -fu

Since switching to a make driven build do_build.sh no longer
does the right thing.  Hack around this by providing a make
target that calls do_build.sh -fu directly.

There are times when you want to update the FreeBSD sources and
rebuild just kernel and or world, but yet use all of the packages
that have been built in the build environment. (Having a global
package cache will reduce wanting this)

In a make driven build process you want something that just updates
the sources then returns so you can move to the next stage of the
build which then does the compile stage and so forth.

Currently do_build.sh -f forces and buildworld/buildkernel, but
then proceeds to build an entire FreeNAS image.  Likewise
do_build.sh -fu updates the sources, then does buildworld/kernel
then builds a FreeNAS image.