bsd.crunchgen.mk: Change paths

This file is intended to be used from src/share/initrd/
and not src/rescue/ like on FreeBSD.
bsd.crunchgen.mk: Add a mechanism to keep handpicked symbols
crunchgen(1): Stop adding underscores to symbols

* The configuration file has provisions to manually specify a few symbols
  to keep public.

* Unfortunately this feature was rendered useless by crunchgen itself,
  which prepended an underscore character before each symbol in the call
  to crunchide(1).
mkinitrd(8): Optimize newfs for size