build.sh now NETBSDSRCDIR/mk.conf if present

I've enhanced build.sh to support using mk.conf in the same directory
(i.e., the top level of the source tree) if present, instead of /etc/mk.conf.
This is not done if $MAKECONF is already set in the environment.

I.e, the mk.conf / MAKECONF lookup order is now, in descending precedence:
	1. MAKECONF in the environment
	2. mk.conf at the top of the source tree, if present
	3. /etc/mk.conf default from share/mk/bsd.own.mk

This means unprivileged users can benefit from mk.conf(5)
semantics without write privileges to /etc/mk.conf, and
also simplifies per-source-directory configuration.

In all cases, MAKECONF is also set in the generated makewrapper
(it previously wasn't), for consistency with other variables.


Luke.