Allow grub to autodetect screen resolution

With this, systems without a supported non-vesa FB driver in Linux
(such as Intel integrated graphics) get full resolution after reboot.

steps/improve/make_bootable.sh

@@ -27,6 +27,7 @@ set default=0
 menuentry 'Linux live-bootstrap (4.9.10)' {
     insmod part_msdos
     set root='$(grub-probe -d /dev/${DISK} -t bios_hints | sed -e 's/ //g')'
+    set gfxpayload=auto
     linux /boot/linux-4.9.10 root=/dev/${DISK} rw $(cat /proc/cmdline)
 }
 EOF