Console: introduce console=none command line parameter

Currenty, not specifying 'console=<foo>' on the command line causes
Xen to default to 'vga'.  Alternativly, the user can explicitly
specifiy 'console=vga|com1|com2'.

However, there is no way to specify that neither vga nor serial should
be used.  Specifying 'console=' does have the effect that neither vga
nor serial is set up, but at the cost of an "Bad console= option ''"
warning.

Therefore, expliticly support a 'console=none' option which does not
set up vga and does not set up serial, but does not trigger the bad
console warning.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

diff -r a2a8089b1ffb xen/drivers/char/console.c
--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -549,6 +549,8 @@ void __init console_init_preirq(void)
             p++;
         if ( !strncmp(p, "vga", 3) )
             vga_init();
+        else if ( !strncmp(p, "none", 4) )
+            continue;
         else if ( strncmp(p, "com", 3) ||
                   (sercon_handle = serial_parse_handle(p)) == -1 )
         {