EC2: Don't use unicode in boot loader

The boot loader menu is disabled by default in EC2, but if it is ever
turned on, the default (unicode) output breaks EC2's web interface to
the serial console.

Set loader_menu_frame="ascii" instead.

MFC after:	3 days
Sponsored by:	Amazon

release/tools/ec2.conf

@@ -41,9 +41,11 @@ ec2_common() {
 	metalog_add_data ./boot.config
 
 	# Booting quickly is more important than giving users a chance to
-	# access the boot loader via the serial port.
+	# access the boot loader via the serial port; but if users turn the
+	# boot loader back on, avoid ASCII since it breaks the EC2 web console.
 	echo 'autoboot_delay="-1"' >> ${DESTDIR}/boot/loader.conf
 	echo 'beastie_disable="YES"' >> ${DESTDIR}/boot/loader.conf
+	echo 'loader_menu_frame="ascii"' >> ${DESTDIR}/boot/loader.conf
 
 	# The EFI RNG on Graviton 2 is particularly slow if we ask for the
 	# default 2048 bytes of entropy; ask for 64 bytes instead.