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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<tr><td><span title='2020-10-13 19:01:33 +0200'>2020-10-13</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=b4d7d41b222e6b8d8e85980e0da85785c86c0966'>gnu: dos2unix: Cross-compile.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-10-13 19:01:33 +0200'>2020-10-13</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=e2d33f9e33b817490768fc723761c6a29e34cb9a'>gnu: dos2unix: Update to 7.4.2.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-09-24 20:06:41 +0300'>2020-09-24</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=3d0c0ae661ccafcab030f27a77179c772039f682'>gnu: java-rsyntaxtextarea: Don't use unstable tarball.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2020-08-13 16:09:41 +0200'>2020-08-13</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=3a176be045c262047892806e3db6335b43d49c1d'>gnu: aha: Cross-compile.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-08-13 16:09:41 +0200'>2020-08-13</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=7d98468cecdf026182c30ced63a44bbbfb361847'>gnu: aha: Update to 0.5.1.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-08-12 21:26:40 +0200'>2020-08-12</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=73092df4686074572e79a54946d9b62f495f58a7'>gnu: libgtextutils: Hide default GCC.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2020-07-12 23:08:15 +0200'>2020-07-12</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=b0e7b6992f3f845e83cfbca4d700b51dba50b4d5'>gnu: Remove ".git" from "https://github/…/….git".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-06-27 20:08:23 +0200'>2020-06-27</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=e36c4d0bca0b41fa24c271f4009aacf866b45f47'>gnu: libconfuse: Do not build static libraries.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-06-27 20:08:23 +0200'>2020-06-27</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=07d6003be0c4d1d631a65f36a88cf69685d6c8b1'>gnu: libconfuse: Update to 3.3.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-05-31 22:12:12 -0400'>2020-05-31</span></td><td><a href='/guix/commit/gnu/packages/textutils.scm?id=7f05493da3521ce4c470935d9b82fbde8e0836c6'>gnu: Add drm-tools.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>