This should have been done as part of rS327350, but due to lack of foresight it came later. In the different places we apply overlays, we duplicate the bits that check for fdt_overlays in the environment and supplement that with any other places we need to check for overlays to load. These "other places" will be loader specific and are not candidates for consolidation.
Provide an fdt_load_dtb_overlays to capture the common logic, allow passing in a callback that can be used to obtain additional overlays to be loaded. The callback is used in practice for ubldr to pull in any fdt_overlays passed to it from U-Boot environment, but it can be used for any other source of overlays.
These additional overlays supplement loader.conf(5) fdt_overlays, rather than replace, so that we're not restricted to specifying overlays in only one place. This is a change from previous behavior where loader.conf(5) supplied fdt_overlays would cause us to ignore U-Boot environment, and this seems nonsensical- user should have sufficient control over both of these aspects, or lack of control for good reasons.
A knob could be considered in the future to ignore U-Boot supplied overlays, but the supplemental treatment seems like a good start.