configs/*/grub/grub.cfg: enable serial input and output

Try to initialize a serial device and use it for input and output.

Add more comments to grub.cfg to explain what is done.

Related to #75

grub/grub.cfg

@@ -1,12 +1,12 @@
+# Load partition table and file system modules
 insmod part_gpt
 insmod part_msdos
 insmod fat
 insmod iso9660
 
+# Use graphics-mode output
 insmod all_video
-
 insmod font
-
 if loadfont "${prefix}/fonts/unicode.pf2" ; then
     insmod gfxterm
     set gfxmode="auto"
@@ -14,14 +14,22 @@ if loadfont "${prefix}/fonts/unicode.pf2" ; then
     terminal_output gfxterm
 fi
 
+# Enable serial console
+if serial --unit=0 --speed=115200; then
+    terminal_input --append serial
+    terminal_output --append serial
+fi
+
+# Set default menu entry
 default=archlinux
 timeout=15
 timeout_style=menu
 
 # GRUB init tune for accessibility
-#
 play 600 988 1 1319 4
 
+# Menu entries
+
 menuentry "Arch Linux install medium (x86_64, UEFI)" --class arch --class gnu-linux --class gnu --class os --id 'archlinux' {
     set gfxpayload=keep
     search --no-floppy --set=root --label %ARCHISO_LABEL%