mvebu: cortexa53: uDPU/eDPU: update active bootscript as well

mvebu: cortexa53: uDPU/eDPU: update active bootscript as well

Currently, sysupgrade will only upgrade the unused slot, however since the
whole dual firmware logic is in the bootscript U-boot will just use the
first bootscript it finds.

So, in a case that you are running slot A it will upgrade slot B, however
that means that slot B will be still booted by the old bootscript that came
with the previous firmware version.

This is an issue if you need to change anything, so lets add a custom
function that upgrades the active bootscript as well after flashing the
slot firmware.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>