Change 'grub-mkrescue' to honor the 'GRUB_FAT_SERIAL_NUMBER'
environment variable.  That way, the caller can specify a fixed
serial number (instead of the randomly chosen one) to create EFI
images (the 'efi.img' file) that are reproducible bit-for-bit.

Patch by Ludovic Courtès <ludo@gnu.org>.

--- grub-2.02/util/grub-mkrescue.c	2019-04-20 19:15:26.180242812 +0200
+++ grub-2.02/util/grub-mkrescue.c	2019-04-20 21:56:34.672370849 +0200
@@ -788,8 +788,15 @@ main (int argc, char *argv[])
 
       efiimgfat = grub_util_path_concat (2, iso9660_dir, "efi.img");
       int rv;
-      rv = grub_util_exec ((const char * []) { "mformat", "-C", "-f", "2880", "-L", "16", "-i",
-	    efiimgfat, "::", NULL });
+
+      const char *fat_serial_number = getenv ("GRUB_FAT_SERIAL_NUMBER");
+      const char *mformat_args[] =
+	{ "mformat", "-C", "-f", "2880", "-L", "16",
+	  fat_serial_number != NULL ? "-N" : "-C",
+	  fat_serial_number != NULL ? fat_serial_number : "-C",
+	  "-i", efiimgfat, "::", NULL };
+
+      rv = grub_util_exec (mformat_args);
       if (rv != 0)
 	grub_util_error ("`%s` invocation failed\n", "mformat");
       rv = grub_util_exec ((const char * []) { "mcopy", "-s", "-i", efiimgfat, efidir_efi, "::/", NULL });
/nix/scripts/offload.in?id=26150eac734cb91c00420d382ff0cb39f391d3a1'>tree</a><a href='/guix/commit/nix/scripts/offload.in?id=26150eac734cb91c00420d382ff0cb39f391d3a1'>commit</a><a href='/guix/diff/nix/scripts/offload.in?id=26150eac734cb91c00420d382ff0cb39f391d3a1'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/nix/scripts/offload.in'>
<input type='hidden' name='id' value='26150eac734cb91c00420d382ff0cb39f391d3a1'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=26150eac734cb91c00420d382ff0cb39f391d3a1'>root</a>/<a href='/guix/log/nix?id=26150eac734cb91c00420d382ff0cb39f391d3a1'>nix</a>/<a href='/guix/log/nix/scripts?id=26150eac734cb91c00420d382ff0cb39f391d3a1'>scripts</a>/<a href='/guix/log/nix/scripts/offload.in?id=26150eac734cb91c00420d382ff0cb39f391d3a1'>offload.in</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/nix/scripts/offload.in?id=26150eac734cb91c00420d382ff0cb39f391d3a1&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>