MFC boot loader path and RBX constant deduplication

r294765 (imp)

Move all the separate copies of the same strings into paths.h. There's
nothing machine specific about these.

r294765 (imp)

RBX_ defines are in rbx.h, move it there.

r294847 (imp)

Remove static from these two. They slipped through the cracks.

r294925 (imp)

Fix mistake when transitioning to the new defines with ZFS loader. I
hate adding yet another define, but it is the lessor of the evil
choices available. Kill another evil by removing PATH_BOOT3 and
replacing it with PATH_LOADER or PATH_LOADER_ZFS as appropriate.

Approved by: re (gjb)

MFC: r268475 (by imp)

Make SERIAL support optional again. Enable it for i386 because a huge
percentage of machines has a 16550. Disable it for pc98 since only a
tiny fraction of them have one.

MFC: r268475 (by imp)

Make SERIAL support optional again. Enable it for i386 because a huge
percentage of machines has a 16550. Disable it for pc98 since only a
tiny fraction of them have one.