Avoid usage of mtdblock

mtdblock is another layer on top of mtd which in most cases (since Linux 3.0 and new busybox) is unnecessary. For reading or mounting FS from mtd is enough to use just mtd device.

For example this file mounts /dev/mtdblock4 with jffs2 filesystem:
https://gitlab.nic.cz/turris/os/packages/-/blob/master/hardware/cert-backup/files/turris1x_backend.sh
Replacing /dev/mtdblock4 by just mtd4 also works fine and avoids usage of unnecessary mtdblock layer.

For mount and read cases simple sed 's/\/dev\/mtdblock/mtd/g' is enough.