components/liveinstall: Enable mediacheck for live ISOs

This brings us to full parity with official Fedora ISOs produced
by Lorax.

grub-ppc.cfg.iso-template

@@ -1,6 +1,6 @@
 # Inspired by the config used for lorax-built live media
 
-set default=${default_boot}
+set default="1"
 
 insmod gzio
 insmod part_gpt
@@ -18,6 +18,10 @@ menuentry "Start ${title}" --class fedora --class gnu-linux --class gnu --class
 	linux ($$root)${bootpath}/${kernel_file} ${boot_options}
 	initrd ($$root)${bootpath}/${initrd_file}
 }
+menuentry "Test this media & start ${title}" --class fedora --class gnu-linux --class gnu --class os {
+	linux ($$root)${bootpath}/${kernel_file} ${boot_options} rd.live.check
+	initrd ($$root)${bootpath}/${initrd_file}
+}
 submenu "Troubleshooting -->" {
 	menuentry "Start ${title} in basic graphics mode" --class fedora --class gnu-linux --class gnu --class os {
 		linux ($$root)${bootpath}/${kernel_file} ${boot_options} $${basicgfx}