bfd: move header updates up a directory

bfd: move header updates up a directory

The rules for rebuilding the bfd headers live in the doc/ subdir
(most likely) because they rely on the chew & related tools.  But
we can collapse them into the main Makefile while keeping the tools
in the doc subdir easily enough.  This makes the code simpler and
allows for rebuilding them in parallel.

Also add automake silent rule support while we're here.