when both boot_a and boot_b is filled with Mobian kernel, boot_a is never flashed

Scenario:

Each time Mobian tries to update the kernel, it detects the boot_b partition so it omits flashing boot_a partition.

This leads to three posibile failure after the boot:

usually, when old kernel modules are kept just old kernel boots without change
when old kernel is removed (in trial to get new flashed) modules won't loads, so user is stuck with broken system
sometimes it (fastboot ?) switches to slot B, which leads to not being able leave fastboot until manual invoking computer $ fastboot --set-active=a

Proposed solution:

detect which slot is selected at fastboot level and flash only that slot (if possible ?)
when Mobian is detected at both partitions, flash them both