feat(coreos-bootkernel) add coreos-bootkernel ebuild

This is a separate kernel package, and configuration, for the "boot"
kernel.  It reuses the same eclass for kernel builds, but does two
things differently for boot kernels:
  - no boot/vmlinuz symlink
  - picks the boot version of the kernel configuration.
fix(coreos-bootkernel) remove commented out line in ebuild

Found during code review, good eye.
Merge pull request #17 from gregkh/bootkernel

feat(coreos-bootkernel) add coreos-bootkernel ebuild