src.sys.mk: Support src.conf in SRCTOP

[des]

Summary

If SRCCONF is not defined and src.conf exists at the top level of the
source tree, use that instead of /etc/src.conf.

MFC after: 3 days

[emaste]

I think this is a good idea

[zlei]

In D52470#1198285, @emaste wrote:

I think this is a good idea

+1

[kevans]

Can we preemptively add /src.conf to .gitignore if we do this?

[imp]

Also agreed with kevans: please add /src.conf to .gitignore

[kevans]

share/mk/src.sys.mk

2

Presumably also want to update the kern.pre.mk version.

[des]

rf

[kevans]

Thanks!