Move libxml2 and libffi earlier in the list of ports to be built.
When I tried to build FreeNAS from a clean tree, I had multiple
failures because libxml2 and libffi did not exist.

Long term, the dependencies in the ports build in FreeNAS needs
to be improved, but this is good enough for now.
(1)  Add a file, nanobsd/DEBUG, which contains kernel config options to enable
     more kernel debugging.

(2)  Take some logic from build/nanobsd/nanobsd.sh which is hardcoded
     for building and installing a single kernel, and reimplement it in
     nanobsd/os-base, but make it more flexible so that you
     can specify a different kernel to build and install.

(3)  In nanobsd/os-base, add logic which appends nanobsd/DEBUG
     to the existing kernel config, to create a new kernel config.
     For example, nanobsd/DEBUG will be appended to nanobsd/FREENAS.amd64
     to create a new kernel config file, FREENAS.amd64-DEBUG.
     The actual generated kernel config file will live in
     ${NANO_OBJ}/, so it will be os-base/amd64/FREENAS.amd64-DEBUG.

(4)  Add logic to install the kernel generated in (3) into
     /boot/kernel-debug/ on the running system.

(5)  Hook the logic for building the debug kernel into the overall build
     logic by using "customize_cmd".
Move building and installing of debug kernels
to before we start adding ports.
When I first tried to build FreeNAS, I encountered multiple
random build errors during the building of ports when
the build tried to unmount certain nullfs file systems.
It turns out that the devel/gamin port installed on my system
(which gets pulled in via the devel/glib20 port) tries
to poll mounted file systems to monitor file system activity, and sometimes
gets stuck, preventing the file system from unmounting.

Add a note to the README explaining how to disable gamin,
to avoid this problem when building FreeNAS.
If we specify  "do_build.sh -x", then print out the
invocation of nanobsd.sh.  It's useful to see what the looks like
when debugging the build.
Merge branch 'master' of git://github.com/rodrigc/freenas into rodrigc-master