efi_max_resolution needed on base/head for UEFI systems without FHD displays

r331464 added efi_max_resolution. If you run base/head on monitors without Full HD (FHD), then you might need to specify your desired resolution in /boot/loader.conf.

This should suffice for a resolution of 1280×1024 on 19 inches displays.

efi_max_resolution="1280x1024"