cmake: add reverse option to `zephyr_get(... MERGE)`

Adding the possibility to have the list returned in reversed order when
using `zephyr_get(... MERGE).

`zephyr_get(... MERGE)` creates a list which populates the content based
on variable settings in the following scopes, in this order:
sysbuild, cache, environment, local.

This works well for lists where content first in list has highest
precedence, such as ROOTs settings.
However, for settings where the value last in the list will overwrite
values earlier in the list, we want the list to be reversed, examples
of such can be CONF_FILE, OVERLAY_CONFIG, DTC_OVERLAY_FILE, where the
content of the file last in the list will overrule the content from an
earlier file.
So to ensure that a DTC_OVERLAY_FILE defined as cache takes precedence
over an env or local scope variable the possibility of reversing the
list must be available.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>