coreos-kernel: populate /lib/modules/$(uname -r)/build

[vcaputo]

With this changeset one can build a kernel module out-of-tree in a dev
image in the usual fashion:

 ~# cat Makefile
 obj-m := noop.o
 ~# make -C /lib/modules/4.3.0-coreos-r1/build M=$PWD
 make: Entering directory '/usr/lib64/modules/4.3.0-coreos-r1/build'
   CC [M]  /tmp/noop/noop.o
   Building modules, stage 2.
   MODPOST 1 modules
   CC      /tmp/noop/noop.mod.o
   LD [M]  /tmp/noop/noop.ko
 make: Leaving directory '/usr/lib64/modules/4.3.0-coreos-r1/build'
 ~#

/lib/modules/$(uname -r)/source is now populated with a stripped source
tree (headers, scripts, Makefiles...), prod images may build out-of-tree
modules by simply bind mounting these directories into a toolchain
container without needing kernel source.

Note that this build directory is being populated from the actual
$KBUILD_OUTPUT used in producing the kernel in the image, which has simply
been make cleaned and stripped down:

 /lib/modules/4.3.0-coreos-r1/build# ls -la
 drwxr-xr-x 5 root root   4096 Nov 25 11:56 .
 drwxr-xr-x 4 root root   4096 Nov 25 11:56 ..
 -rw-r--r-- 1 root root 108265 Nov 25 11:52 .config
 -rw-r--r-- 1 root root      2 Nov 25 11:55 .version
 -rw-r--r-- 1 root root    578 Nov 25 11:54 Makefile
 -rw-r--r-- 1 root root 617170 Nov 25 11:55 Module.symvers
 drwxr-xr-x 3 root root   4096 Nov 25 11:23 arch
 -rw-r--r-- 1 root root  23280 Nov 25 11:52 defconfig
 drwxr-xr-x 4 root root   4096 Nov 25 11:23 include
 drwxr-xr-x 6 root root   4096 Nov 25 11:56 scripts
 lrwxrwxrwx 1 root root      9 Nov 25 11:56 source -> ../source
 /lib/modules/4.3.0-coreos-r1/build#

[vcaputo]

This PR uses coreos/linux#3, so lets land that first.

[vcaputo]

@marineam @mjg59 PTAL

[crawford]

Addresses coreos/bugs#949.

[marineam]

This would probably be more readable in a function. I can't think of a need for the subshell otherwise.

[vcaputo]

@marineam think we can land this as-is or do you see other things you'd like changed?

[marineam]

LGTM