package/mtd: disable crypto for host

Commit f1d1347b added the optional
openssl dependency for mtd however nothing was done for host-mtd

As a result, crypto support on host-mtd depends on the availability of
host-openssl which is not listed in HOST_MTD_DEPENDENCIES

Moreover, crypto support needs host linux-headers as it includes
<linux/hash_info.h> since version 2.1.2 and
https://github.com/sigma-star/mtd-utils/commit/a739b59efe7996e3bdcbe8b17743dc05ac7c110a

So just disable crypto support for host-mtd

Fixes:
 - http://autobuild.buildroot.org/results/e581bd04e1902476afcdc9d9e34f5fb00fcd31be



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>