Both boot and boot-conf were using a different algorithm from the one
used by start to find the kernel. Fix this.

Also, boot would proceed immediately in the absence of a path as
argument. Check first if a kernel has already been loaded, and, if
not, fall back to load kernel&modules behavior.

Some further factorizing. I deem this code to be mostly readable by
now! :-)

Many thanks to: Makoto MATSUSHITA <matusita@jp.FreeBSD.org>