universe: More gracefully handle architectures without kernels

[jhb]

Summary

Only define the universe_<target>_kernels target that wraps
universe_kernels for a given architecture if the architecture's kernel
config directory exists.

[jhb]

Today you just get a warning in the make output that the command line to generate the list of kernel configs fails. This is nicer as it avoids not just the warning but also doesn't claim to enter the kernels stage only to build nothing.

[imp]

Clever way to know.