Set absolute filename in files like
    /usr/share/qemu/firmware/50-edk2-x86_64-secure.json

Bug: https://bugs.gentoo.org/766743
Bug: https://bugs.launchpad.net/qemu/+bug/1913012
Patch-by: Jannik Glückert
--- a/pc-bios/descriptors/meson.build
+++ b/pc-bios/descriptors/meson.build
@@ -9,7 +9,7 @@ if install_edk2_blobs
   ]
     configure_file(input: files(f),
                    output: f,
-                   configuration: {'DATADIR': qemu_datadir},
+                   configuration: {'DATADIR': get_option('prefix') / qemu_datadir},
                    install: get_option('install_blobs'),
                    install_dir: qemu_datadir / 'firmware')
   endforeach