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>.
Mangled (for GRUB 2.04) by Tobias Geerinckx-Rice <me@tobias.gr>.

--- grub-2.04/util/grub-mkrescue.c	2019-05-20 13:01:11.000000000 +0200
+++ grub-2.04/util/grub-mkrescue.c	2019-07-08 23:57:36.912104652 +0200
@@ -809,8 +809,15 @@
       free (efidir_efi_boot);
 
       efiimgfat = grub_util_path_concat (2, iso9660_dir, "efi.img");
-      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 });
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<div class='path'>path: <a href='/guix/log/?id=53c1e4316d97e794ef015c1fd16bf987c9d595ed'>root</a>/<a href='/guix/log/etc?id=53c1e4316d97e794ef015c1fd16bf987c9d595ed'>etc</a>/<a href='/guix/log/etc/disarchive-manifest.scm?id=53c1e4316d97e794ef015c1fd16bf987c9d595ed'>disarchive-manifest.scm</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/etc/disarchive-manifest.scm?id=53c1e4316d97e794ef015c1fd16bf987c9d595ed&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-08-06 23:14:41 +0200'>2022-08-06</span></td><td><a href='/guix/commit/etc/disarchive-manifest.scm?id=eab5366e679c149cf0a5abeb9a196700b171d9da'>disarchive-manifest: Exclude the Chromium tarball.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-08-05 15:28:02 +0200'>2022-08-05</span></td><td><a href='/guix/commit/etc/disarchive-manifest.scm?id=6d4a0ccecb58c62a95f7f36fa2f4fa53dd52312e'>disarchive-manifest: Filter out origins without a hash.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-08-05 00:14:29 +0200'>2022-08-05</span></td><td><a href='/guix/commit/etc/disarchive-manifest.scm?id=2acffd56a58c01785ea6ea738b79991c0685d364'>disarchive-manifest: Handle tar.xz archives.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-10-09 01:02:18 +0200'>2021-10-09</span></td><td><a href='/guix/commit/etc/disarchive-manifest.scm?id=2ad1ef844632b7605ad6dd362e947a122a0a3925'>disarchive-manifest: Provide valid 'version' string.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-09-14 11:50:58 +0200'>2021-09-14</span></td><td><a href='/guix/commit/etc/disarchive-manifest.scm?id=98712c14fe3517a48603a12eb2e64db305a54fe5'>etc: Add 'disarchive-manifest.scm'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>