init.sh: make sure mountpoints exist

/emmc and /sdcard are only required for the BML to MTD
conversion, create them only when necessary.

/emmc will be removed from the initramfs, hence rmdir /emmc is
no more necessary.

Also, it shouldn't be necessary to keep /emmc and /sdcard mounted,
recovery should mount them automatically if required, but there's
no reason to mount, umount and then mount again the needed block.