kernel/bbclass: rework kernel and module classes to allow for building out-of-tree modules

The existing infrastructure uses an external build tree which references the
kernel source in the work dir. If run with rm work, building external modules
will fail.

This patch places a configured source tree in sysroots. Striking a balance
between minimal size and minimal maintenance is difficult. A fully configured
tree is about 500MB after a clean. This version leans on the side of caution and
removes only the obviously unecessary parts of the source tree to conserve
space, resulting in about 170MB. The arch directories would be some additional
pruning we could do. Given examples from the devel package from distributions, I
suspect this size could be reduced to 75MB or so, but at the cost of a much more
complex recipe which is likely to require a great deal more maintenance to keep
current with kernel releases.

Care is also taken to clean the hostprogs in scripts, and the modules are
responsible for building them as needed. Although it is unclear to me if this is
really necessary, especially considering that modules put these bits back as
soon as they compile. If we are not generating an sstate package, I suspect we
can ignore these.

Please try this with your modules and let me know how it does. I tried to take
non linux-yocto kernel recipes into account, but I have only tested with
linux-yocto and the hello-mod recipe so far.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Acked-by: Koen Kooi <koen@dominion.thruhere.net>
Acked-by: Gary Thomas <gary@mlbassoc.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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