fix(sys-kernel/coreos-kernel): Drop old no-modules kernel.
fix(cros-workon.eclass): Fix support for EAPI=5 subslots

The subslot should not be used as part of package name directories.
This fixes the kernel builds now that cros-kernel2 uses subslots.
fix(sys-kernel/coreos-kernel): Don't install firmware to root filesystem

In preparation for moving to using firmware provided by the
linux-firmware repository instead of linux the kernel ebuild needs to
stop installing the files to avoid conflicts. Also to better ensure that
the firmware package gets rebuilt every time the kernel does bump to
EAPI=5 and set the subslot to the ebuild version/revision. The firmware
package can then depend on the kernel w/ a special slot operator to make
sure it gets rebuilt when the kernel version changes. The firmware
package can then scan the installed modules and only install the
firmware that is required.

(Portage automatic rebuild behavior often makes this sort of rebuild
happen anyway but using subslots makes it a strict requirement.)
add(sys-kernel/coreos-firmware): Add firmware package.

This provides firmware from linux-firmware but excludes everything not
explicitly required by coreos-kernel. Note that firmware installed by
this will only be available on the root filesystem, the initrd still
uses the smaller set of firmware the linux repo provides.

The current 3.12 kernel wants a few files missing in the July snapshot:

 * Missing firmware: ct2fw-3.2.1.1.bin (bna.ko)
 * Missing firmware: ctfw-3.2.1.1.bin (bna.ko)
 * Missing firmware: cxgb4/t5fw.bin (cxgb4.ko)

These files have been added to the linux-firmware repo so a newer
snapshot will be required but for now I'm sticking with what is already
in Gentoo so I don't have to generate my own tarball or whatever.
Merge pull request #291 from marineam/firmware

Moar Firmware!