Revert "Remove base/syslinux and base category"

This reverts commit 217601d933a27f141008de4a6ec0be2ac906b380.

Oops. Heh. Let's play with SYSLINUX some more...
Remove the hacky and unused tag-tracking support
Update linux-kernel.exlib

- Github changed their raw URI for some reason
- Try to solve a bash bug (?) by removing edo in pkg_* phases
- Move pkg_prerm to pkg_postrm, which seems more correct
Update kernel optimisation patch to unstable

Important: this requires GCC 4.9! Although it *might* not matter much if
you don't have a recent Intel CPU (UNTESTED!).
Go back to the 3.15+ branch of kernel gcc patch

The unstable branch doesn't apply.
Re-add edo (sadly it doesn't make it work)

Also remove debugging aid in pkg_postinst.
Major changes to sys-boot/grub