boot/grub2: Set terminfo dumb

GRUB seems to have trouble with line drawing 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:
 
+terminfo dumb
+
 load_env
 regexp --set 1:disk '(.*),.*' $root
 for dev in ($disk,gpt*); do