hooks/{instsoft,updatebase}.GRMLBASE: be more careful about BUILD_ONLY environment.

If BUILD_ONLY is available in the environment then we do not want
to update any packages but just build the ISO.  The script
instsoft.GRMLBASE is being executed *after* updatebase.GRMLBASE
and we run "skiptask instsoft" from updatebase.GRMLBASE. But this
might fail if the script exists too early due to errors.

This commit adds some further checks and also checks for
BUILD_ONLY inside instsoft.GRMLBASE.

Quoting ~log/fai/grml/softupdate-20101227_181604/fai.log from a failed execution:

  [...]
  Calling hook: updatebase.GRMLBASE
  Action softupdate of FAI (hooks/updatebase.GRMLBASE) via grml-live running
  mount: proc already mounted or /linuxuser/2010.12//grml_2010.12-rc1/grml_chroot/proc busy
  mount: according to mtab, proc is already mounted on /linuxuser/2010.12/grml_2010.12-rc1/grml_chroot/proc
  updatebase.GRMLBASE  FAILED with exit code 32.
  Skiping task_updatebase
  Calling hook: instsoft.GRMLBASE
  Action softupdate of FAI (hooks/instsoft.GRMLBASE) via grml-live running
  Get:1 http://deb.grml.org grml-stable Release.gpg [197 B]
  [...]