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.

[siva]

Anything else here? Should be good to go.

[imp]

The config too old warning likely should go too, but that's not the scope of this.

[lwhsu]

IN general direction looks good to me: for a multi-target universe build with a shared KERNCONFS list, a KERNCONF that does not exist for a particular TARGET should not abort the entire build.

If I can do some nitpicking, we can distinguish the config missing case from other config(8) failures. An empty TARGET_ARCH_${kernel} can also mean that the KERNCONF exists but is invalid, has a bad include, has a malformed/unsupported machine directive, etc. I think only a warning for that would make universe build miss to catch broken in-tree kernel config.

Maybe we keep the warning only for the missing KERNCONF case, e.g. !exists(${KERNSRCDIR}/${TARGET}/conf/${kernel}), and keep emitting an error when the file exists but config -m still cannot determine TARGET_ARCH? i.e., something like this (not tested):

.if empty(TARGET_ARCH_${kernel})
.    if !exists(${KERNSRCDIR}/${TARGET}/conf/${kernel})
.        warning ${TARGET}/conf/${kernel} missing; skipping.
.    else
.        error Target architecture for ${TARGET}/conf/${kernel} may be invalid.
.    endif
.endif

[siva]

@lwhsu How about this now?