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

Makefile

@@ -758,12 +758,16 @@ KERNCONFS!=	cd ${KERNSRCDIR}/${TARGET}/conf && \
 		${_THINNER}
 universe_kernconfs: universe_kernels_prologue .PHONY
 .for kernel in ${KERNCONFS}
+.if !exists(${KERNSRCDIR}/${TARGET}/conf/${kernel})
+.warning ${TARGET}/conf/${kernel} missing; skipping.
+.else
 TARGET_ARCH_${kernel}!=	cd ${KERNSRCDIR}/${TARGET}/conf && \
 	env PATH=${HOST_OBJTOP}/tmp/legacy/bin:${PATH:Q} \
 	config -m ${KERNSRCDIR}/${TARGET}/conf/${kernel} 2> /dev/null | \
 	grep -v WARNING: | cut -f 2
 .if empty(TARGET_ARCH_${kernel})
-.error Target architecture for ${TARGET}/conf/${kernel} unknown.  config(8) likely too old.
+.error Target architecture for ${TARGET}/conf/${kernel} cannot be determined, kernconf file may be invalid!
+.endif
 .endif
 universe_kernconfs_${TARGET_ARCH_${kernel}}: universe_kernconf_${TARGET}_${kernel}
 universe_kernconf_${TARGET}_${kernel}: .MAKE