mkarchiso: use FAT32 as early as possible

mkfs.fat selects FAT32 for file systems of at least 512 MiB size, but a
FAT32 file system can already be created at 36 MiB size (assuming 512
byte logical sector size).

CHANGELOG.rst

@@ -21,6 +21,7 @@ Changed
 - Use ``xz -9e`` as the releng profile's initramfs compression. Now that mkinitcpio does not decompress the loadable
   kernel modules and firmware files anymore and moves them to the early uncompressed initramfs, we can compress the main
   initramfs image with a higher compression without it having much impact on the ISO build time.
+- Format the EFI system partition image as FAT32 if the size allows it (i.e. if it is at least 36 MiB).
 
 Deprecated
 ----------

archiso/mkarchiso

@@ -527,6 +527,7 @@ _make_boot_on_fat() {
 _make_efibootimg() {
     local imgsize_kib="0"
     local imgsize_bytes=${1}
+    local mkfs_fat_opts=(-C -n ARCHISO_EFI)
 
     # Convert from bytes to KiB and round up to the next full MiB with an additional 8 MiB for reserved sectors, file
     # and directory entries and to allow adding custom files when repacking the ISO.
@@ -536,6 +537,13 @@ _make_efibootimg() {
             function mib_to_kib(x){return x*1024}
             END {print mib_to_kib(ceil((byte_to_kib($1)+8192)/1024))}' <<<"${imgsize_bytes}"
     )"
+
+    # Use FAT32 as early as possible. mkfs.fat selects FAT32 if the size ≥ 512 MiB, but a FAT32 file system can already
+    # be created at 36 MiB size (assuming 512 byte logical sector size).
+    if (( imgsize_kib >= 36864 )); then
+        mkfs_fat_opts+=(-F 32)
+    fi
+
     # The FAT image must be created with mkfs.fat not mformat, as some systems have issues with mformat made images:
     # https://lists.gnu.org/archive/html/grub-devel/2019-04/msg00099.html
     rm -f -- "${efibootimg}"
@@ -543,9 +551,9 @@ _make_efibootimg() {
     if [[ "${quiet}" == "y" ]]; then
         # mkfs.fat does not have a -q/--quiet option, so redirect stdout to /dev/null instead
         # https://github.com/dosfstools/dosfstools/issues/103
-        mkfs.fat -C -n ARCHISO_EFI "${efibootimg}" "${imgsize_kib}" >/dev/null
+        mkfs.fat "${mkfs_fat_opts[@]}" "${efibootimg}" "${imgsize_kib}" >/dev/null
     else
-        mkfs.fat -C -n ARCHISO_EFI "${efibootimg}" "${imgsize_kib}"
+        mkfs.fat "${mkfs_fat_opts[@]}" "${efibootimg}" "${imgsize_kib}"
     fi
 
     # Create the default/fallback boot path in which a boot loaders will be placed later.