yocto-postinstallation: calculate a path to boot partition if corresponding variable isn't set

During the testing of the latest improvements for WIC image, which enable AB-partitioning schema for i.MX8, it was discovered that system boots with basic dtb (without applying overlays) even on the second boot (when post-installation scripts are already executed). And further investigation has revealed that the post-installation scripts use the wrong boot partition when they are launched from the B-slot.

To fix the problem, implemented a routine to autodetect the boot partition with findmnt, when BOOTPARTITION is not provided to post-installation scripts.