Boot manager/loader

[jhuntwork]

Up to now, mere has used syslinux which worked great. I chose that because it was simple, relatively straightforward, while still supporting a lot of options and configurations.

But syslinux seems to be no longer maintained, the last release was in 2014. That alone might be fine if the software just works, but it also seems its compilation has some pretty tight dependencies on gcc, which we dropped in favor of llvm/clang.

So there's a few choices here:

1. Punt this decision and use the previously compiled version of syslinux already in the repo, not worrying for now about re-compiling it. Re-visit this issue at a later date.

2. Add gcc back to the repo to stand alongside llvm as an optional compiler to support syslinux and any other programs like it with a gcc dependency.

3. Choose and add a new boot loader/manager, probably either grub2 or refind

[jhuntwork]

Choosing option #2 at the moment. I prefer syslinux as it is lightweight, simple to use, and affords a lot of flexibility. Adding what is necessary to make it build.