mtd-utils/flash_eraseall: tiny cleanup

[Sebastian Andrzej Siewior]

tiny cleanup:
- #1 is almost a pure code move, except that now it is non fatal if a
  clean marker can't be prepared.
- #2 add support/ignore for flash types which don't support clean markers
- #3 is tricky. At this point we don't really which format is expected by
  the kernel. So we could either add another command line switch or get
  rid of this option :) Any comments on this?


Sebastian