armsr: change image names to 'combined-efi'

armsr: change image names to 'combined-efi'

luci-app-attendedsysupgrade expects images to be named
'combined-efi' when the system is using EFI images.

This came about as x86 has 'combined' images for legacy
(BIOS) boot and 'combined-efi' for EFI systems.

armsr images were originally named 'combined' only
as there was no 'legacy' image type.

To avoid special handling in the attendedsysupgrade
code, name EFI images consistent with other targets.

Signed-off-by: Mathew McBride <matt@traverse.com.au>
Link: https://github.com/openwrt/luci/pull/6430
Link: https://github.com/openwrt/openwrt/pull/12963
Signed-off-by: Petr Štetiar <ynezz@true.cz>