board-qemu: Add a possibility to use hvterm input instead of USB keyboard

SLOF automatically switches to USB keyboard input when it detects that
a graphics card and USB keyboard are available. However, for debugging
VGA or USB related issues, or running QEMU with the "-nographic" parameter,
it's more convenient to continue using the hvterm as input.
This patch adds the possibility to continue using hvterm if the user
sets the "direct-serial?" NVRAM variable to true. "direct-serial?" is
currently unused for board-qemu -- it's only used on board-js2x so far
for switching the serial ports there, so using this variable for
selecting the serial input on board-qemu seems to be a good fit, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

board-qemu/slof/OF.fs

@@ -231,9 +231,16 @@ romfs-base 400000 0 ' claim CATCH IF ." claim failed!" cr 2drop THEN drop
         " screen" find-alias dup IF nip THEN
         " keyboard" find-alias dup IF nip THEN
         AND IF
-            ." using screen & keyboard" cr
-            " screen" output
-            " keyboard" input
+            ." using screen "
+            s" direct-serial?" evaluate IF
+                ." & hvterm"
+                s" hvterm" input
+            ELSE
+                ." & keyboard"
+                s" keyboard" input
+            THEN
+            cr
+            s" screen" output
         ELSE
             " hvterm" find-alias IF
                 drop