fix(cros-kernel2.eclass): Add kernel modules to initramfs.

Since we need to both bundle modules into the initramfs as well as
bundle the initramfs into the kernel image we need to update a pre-built
image with the user space tools as part of the kernel build process.

This seemed the best scheme, the alternatives were:
 - Unpack bootengine.cpio to a temporary directory, build and install
   kernel modules into that temporary directory, pass that directory
   plus a config file listing what device nodes to the kernel build.
 - Build kernel modules and generate a fresh bootengine.cpio using the
   update-bootengine tool. This would require calling sudo (and breaking
   out of the sandbox in the process) in the middle of the ebuild.
fix(cros-kernel2.eclass): Install vmlinux into same path as modules

This matches the behavior of Debian/Fedora closer so might as well
change it while I'm mucking around in this eclass.
fix(cros-kernel2.eclass): Explicitly build bzImage in second pass

Build only bzImage instead of the default 'all' which also will re-link
all of the modules which is pointless and just causes the initramfs and
rootfs to become out of sync.
fix(cros-kernel2.eclass): Disable OUTOFTREE_BUILD when src is dirty.

If OUTOFTREE_BUILD=1 (currently disabled but that will change) the
kernel will use the existing checkout as the source tree rather than
cloning another tree. If someone built anything in that tree and .config
exists the build will fail complaining that the source isn't clean.
Instead of failing just disable OUTOFTREE and emit a warning.

OUTOFTREE combined with INCREMENTAL makes successive kernel builds
pretty much no-ops (just re-linking modules and bzImage).
fix(cros-kernel2.eclass): Fix STRIP_MASK and disable binchecks

STRIP_MASK wasn't updated to the new vmlinux location earlier.
RESTRICT=binchecks disables checks that only make sense for user space
binaries. This silences a bogus scanelf error.
bump(sys-kernel/coreos-kernel): New modular kernel.

Lots of changes here, some things of note:
 - Switch from built-in to modules for much of the kernel.
 - Enable more features, mostly in power management and networking.
 - Remove no_firmware.patch, this was added back when we had two
   different kernels which would conflict if both installed firmware.
 - Stop disabling OUTOFTREE_BUILD now that we don't patch the source.

So far this is just compile tested so it is marked ~amd64 for now.
fix(cros-kernel2.eclass): Move build deps from ebuilds to eclass.