modutils: fix config options dependency (2)

modutils: fix config options dependency (2)

- The modprobe-small implementation of rmmod no longer chdir's to
  "/lib/modules/`uname -r`" as it was not necessary for rmmod's
  operation. (And it no longer need to die if such modules directory
  doesn't exist.)
- Configs DEFAULT_MODULES_DIR and DEFAULT_DEPMOD_FILE no longer depend
  on MODPROBE_SMALL as the latter may not enable depmod or modprobe
  that requires these configs.
- Clarify DEFAULT_DEPMOD_FILE's description regarding the ".bb" name
  suffix.

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>