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 clean`ed 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#
```
Rather than building the initramfs in the postinst of bootengine, this
moves it to the setup of coreos-kernel. This has the advantage of
failing a build if dracut fails and ensuring that the latest initramfs
makes it into every kernel build.
in https://github.com/coreos/coreos-overlay/pull/1639, the script would
set USE_SELINUX to the blank string, causing an extra blank argument to
be passed to docker daemon, resulting in it not starting when docker
runs on btrfs.
