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

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.

Reviewed by: imp
Approved by: lwhsu (mentor), emaste (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D55571