Rules 'n' things

Follow-up of #1276. Some fixes, some new functionality.

The `kernel_required` rule passes on information from a sub-make to the top level. A check file is created on each level if REQ_KERNEL is set, and is passed on from level to level, until it ends up at the top level.
The downside is the rules don't look great, but I'm not sure there's a way around that.

x64-related: the evansport arch is not included in Synology's x64 arch, probably because it's 32-bit (?).

I'm not sure touching files is the proper way to do that. What about making the rule exit 1 when kernel is required and 0 otherwise and then check the output of the `$(MAKE) that-rule` instead of depending on it?