universe: emit warning instead of error for bad/missing KERNCONF

[siva]

Summary

When doing a large make universe build with multiple KERNCONFS,
it should not be an error when a particular target has a missing
KERNCONF.

In this example,

$ make universe TARGETS='arm64 riscv' KERNCONFS='QEMU VIRT'

Currently, arm64 does not have a QEMU conf, and riscv
does not have a VIRT conf. However, this command should still
succeed instead of failing with the following message:

make[2]: /usr/src/Makefile:767: Target architecture for riscv/conf/VIRT unknown.  config(8) likely too old.
	in .for loop from /usr/src/Makefile:761 with kernel = VIRT
	in make[2] in directory "/usr/src"

make[2]: stopped making "universe_kernels" in /usr/src

Since the opposite is true, we should build whatever confs are available
from the requested list of KERNCONFS, while making sure to still
emit warnings along the way.

[siva]

Any comments on this change?

[imp]

This is likely OK. We can lose the config(8) too old part. That version is now unsupported (11 or 12 had it).

[siva]

Applied feedback in v2.