Adds a check for local-vars for boot_timeout

Closes: https://github.com/netbootxyz/netboot.xyz/issues/1482

roles/netbootxyz/templates/local-vars.ipxe.j2

@@ -15,3 +15,6 @@
 
 ### Sets the local live endpoint for the users local installation
 #set live_endpoint http://my_ip_address:8080
+
+### Sets the default boot timeout
+#set boot_timeout 300000

roles/netbootxyz/templates/menu/boot.cfg.j2

@@ -23,7 +23,7 @@ set sigs {{ sigs_location }}
 iseq ${platform} efi && set ipxe_disk netboot.xyz-snponly.efi || set ipxe_disk netboot.xyz-undionly.kpxe
 
 # set default boot timeout
-set boot_timeout {{ boot_timeout }}
+isset ${boot_timeout} || set boot_timeout {{ boot_timeout }}
 
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