mtd: Allow to select the host variant

If any of the post-image scripts wants to create mtd/ubi images,
we need to expose an option for mtd-tools to be user-selectable.

[Peter: fix include order]
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>