add(sys-boot/syslinux): Custom ebuild with better dependencies.

- Depend on util-linux
- Optionally depend on perl, don't install perl scripts when disabled.
fix(coreos-devel/sdk-depends): Install debinutils in SDK

The kernel's "make install" step requires /sbin/installkernel provided
by this package to work properly.
Merge pull request #416 from marineam/deps

Dep fixes