package/mtd: crypto support needs headers >= 4.12

crypto support includes <linux/hash_info.h> since version 2.1.2 and
https://github.com/sigma-star/mtd-utils/commit/a739b59efe7996e3bdcbe8b17743dc05ac7c110a

However, linux/hash_info.h is only exported since kernel 4.12:
https://github.com/torvalds/linux/commit/fcc8487d477a3452a1d0ccbdd4c5e0e1e3cb8bed

Fixes:
 - http://autobuild.buildroot.org/results/dbaeed9e9e2af191166808d2d6d6d3c2e5051c2c

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

package/mtd/mtd.mk

@@ -21,7 +21,8 @@ endif
 ifeq ($(BR2_PACKAGE_MTD_UBIFS_UTILS),y)
 MTD_DEPENDENCIES += util-linux zlib lzo host-pkgconf
 MTD_CONF_OPTS += --with-ubifs
-ifeq ($(BR2_PACKAGE_OPENSSL),y)
+# crypto needs linux/hash_info.h
+ifeq ($(BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_12)$(BR2_PACKAGE_OPENSSL),yy)
 MTD_DEPENDENCIES += openssl
 MTD_CONF_OPTS += --with-crypto
 else