fstab can't mount the partitions on the first boot if the mountpoint does not
exist. Create the mountpoints at build time so that fstab can mount the
partitions on the first try. Required for the next commits with customizations
for SDMFG.

Also: Having the mountpoints pre-created is beneficial for devices with
read-only root file systems (like MTR with mPower). This solution is actually
ported from mPower:
https://gitlab.multitech.net/multitech/meta-mts-device/blob/0f8b5467c662a1be59a2ee5adc91b7c99aa87497/recipes-core/base-files/base-files_3.0.14.bbappend.

Commit 3908aa3d25d583420d36976bc1a2f11f38b5410d removes the fstab manipulation.
In mPower the configuration partition is selected during boot time from
config.init, without modifying fstab, which in turn saves the flash erase
cycles by not making nay changes on rootfs/overlay.

This commit updates the fstab to permanently use the first configuration
partition as /var/config. The second partition is not used in mLinux.
This approach is similar to the one for mLinux on MTCDT3.

If one needs a config partition switchover on mLinux - please implement it in
a similar manner to mPower, wihout making changes on rootfs during boot.