test-efi-create-disk.sh: allow running from separate build dir, hook up to meson

This allow test-efi-disk.img to be created under meson.
The invocation of qemu is not converted yet, in particular because the
command-line used in Makefile.am is outdated.

src/boot/efi/meson.build

@@ -158,7 +158,7 @@ if have_gnu_efi
          no_undefined_symbols,
          args : [so])
 
-    custom_target(
+    stub = custom_target(
       tuple[1],
       input : so,
       output : tuple[1],
@@ -175,5 +175,18 @@ if have_gnu_efi
                 ['@INPUT@', '@OUTPUT@'],
       install : true,
       install_dir : bootlibdir)
+
+    set_variable(tuple[0].underscorify(), so)
+    set_variable(tuple[0].underscorify() + '_stub', stub)
   endforeach
 endif
+
+############################################################
+
+if have_gnu_efi
+  test_efi_disk_img = custom_target(
+    'test-efi-disk.img',
+    input : [systemd_boot_so, stub_so_stub],
+    output : 'test-efi-disk.img',
+    command : [test_efi_create_disk_sh, '@OUTPUT@', '@INPUT0@', '@INPUT1@', splash_bmp])
+endif