linux-yocto: Add missing mtd_block config

ActsJFFS2Test it is requires mtd_block device to mount jffs2 fs image.

Signed-off-by: Robert Drab <robert.drab@huawei.com>

meta-ohos-acts/recipes-kernel/linux/linux-yocto/0001-fragment-jffs2-tests.cfg

 CONFIG_MTD=y
+CONFIG_MTD_BLOCK=y
 CONFIG_MTD_BLOCK2MTD=m
 CONFIG_JFFS2_FS=y