aespipe: add host variant

Having the host version may allow post-image scripts to encrypt the
resulting image with aespipe (and decrypt it in the update tool on the
target).

Signed-off-by: Ariel D'Alessandro <ariel@vanguardiasur.com.ar>
[Thomas: rewrap Config.in help text, tweak commit title.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>