grub: Force console output

GRUB seems to have trouble with box drawing over the PL011 UART on both
the Raspberry Pi and QEMU.

boot/grub2/grub.cfg

@@ -1,6 +1,8 @@
 # vim: set ft=sh :
 # vim: set sw=4 ts=4 sts=4 et:
 
+terminal_output console
+
 load_env
 regexp --set 1:disk '(.*),.*' $root
 for dev in ($disk,gpt*); do