the more build knobs bikeshed

[Sam Leffler]

Here is a patch against HEAD to add several knew MK_* knobs that can be 
used to trim the build for small-footprint applications:

http://www.freebsd.org/~sam/build.patch

With these changes I can build a nanobsd image for x86 or arm that is 
<1/2 the size you can get otherwise (using just the standard knobs).  If 
you do not use the knobs you should get exactly what you're used to getting.

My goal in doing this is to make nanobsd more useful for building 
embedded packages without bypassing the build process to purge stuff.  
This is still a LONG way from small enough to fit in many applications 
but seems like a useful start.

FWIW I chose knobs using two criteria:

1. Does it save an appreciable amount of space.
2. Does it remove applications that you really don't want to be present.

I'm sure folks will wonder about some choices.  One can also argue that 
some of the groups I've added are wrong and/or we should have individual 
knobs for each application.

Separate issues that I intend to work on:

1. Build time is way too long; paring down the target config should also 
reduce the build time but it does not because things like MK_TOOLCHAIN 
cannot be used when building; only when installing.
2. Some applications that you want are hugely bloated and may benefit 
from some TLC.  bind comes to mind where I can crunchgen all the bits 
into a single binary but when built normally you get multiple huge 
executables.
3. Cross-build and package; I can use nanobsd to build arm images but 
the packaging facilities are inadequate.  I've got a start on default 
setups for some boards/packages and would like to see this grow into a 
library that people can use for reference and/or extend.

Anyway,  I'm interested in getting the above patch committed so looking 
for constructive comments.  I haven't included the files that go under 
src/tools/build/options (I've done them already).  I also have mods for 
RELENG_7 that I've been using for a while.

    Sam