unification of boot.cfg parsing copies

[Tyler Retzlaff <rtr%netbsd.org@localhost>]

hello,

presently there are three not-so-exact copies of code for parsing boot.cfg
across i386, sparc and zaurus.

linked is a patch that attempts to unify the three copies of this code
into a single mi file (bootcfg.c).

the primary source for the parsing logic was taken from i386 and had
minor tweaks made to it to support what was being done in both zaurus
and sparc.  notably the ability to limit the size of the configuration
to be buffered and simply handle and add arch specific with generic
fallback key=value directives.

any issues with the parsing logic as they stand have not been addressed      
as it seemed more reasonable to reduce the number of copies down to
1 before fixing such issues.  some name cleanups have been made since
some of the original naming was ambiguious and not grep friendly
i.e. 'bootconf' -> 'bootcfg'.

note that the previous boot.cfg format used for sparc64 was undocumented
and not compatible.  discussion with martin@ identified that boot.cfg
was likely only used by him and it was satisfactory to align it with i386.

martin@ has been kind enough to test on sparc64 with no obvious defects
and i am also using it on amd64, if someone has a zaurus feel free to
step forward.

http://www.netbsd.org/~rtr/bootcfg0.diff

thanks