ubifs filesystem image: line up and improve Config.in options

ubifs filesystem image: line up and improve Config.in options

Triggered by the 'some comment required here', this patch updates the Config.in
comments for the ubifs filesystem image options. This includes:
- 'erase block' --> 'eraseblock' (following mtd online documentation)
- remove 'UBI' prefix to make options uniform. Sometimes the option is not a
  property of UBI but rather of the flash. Also, line up the name of the option
  with the help of mkfs.ubifs.
- add help text on all options, referring to the mkfs.ubifs/ubinize options

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>