package/grub: Fix grub.cfg

When trying qemu-x86_64 target with full disk image, grub wasn't able to
read the rootfs. After comparing it's config with the one used on my
notebook, I was able to get it right.

Apart from fixing the failure, this also enables serial output so grub
menu becomes accessible when calling qemu with -nographic option.

Signed-off-by: Phil Sutter <phil@nwl.cc>

package/grub/files/grub.cfg

@@ -1,8 +1,14 @@
+serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1
+terminal_input serial
+terminal_output serial
+
 set default=0
 set timeout=3
 
 menuentry "OpenADK" {
+  insmod part_msdos
+  insmod ext2
+  set root='hd0,msdos1'
   echo "Loading OpenADK"
-  set root=(hd0,2)
-  linux (hd0,2)/boot/kernel root=/dev/sda2 rootfstype=ext4 rootwait panic=10
+  linux16 /boot/kernel root=/dev/sda1 rootfstype=ext4 rootwait panic=10
 }