mtd: Add a more verbose menu text

mtd-utils also contain ubi/ubifs tools, so let's add it to the menu option.
Also, add an URL to stick to the help message standard.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>