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@citrix.com>
Committed-by: Keir Fraser <keir@xen.org>
xen-unstable changeset: 24614:f8c2cf24a26c
xen-unstable date: Sat Jan 28 13:41:42 2012 +0000

     1.1 --- a/xen/drivers/char/console.c	Tue Jan 31 11:44:57 2012 +0000
     1.2 +++ b/xen/drivers/char/console.c	Tue Jan 31 11:45:37 2012 +0000
     1.3 @@ -559,6 +559,8 @@ void __init console_init_preirq(void)
     1.4              p++;
     1.5          if ( !strncmp(p, "vga", 3) )
     1.6              vga_init();
     1.7 +        else if ( !strncmp(p, "none", 4) )
     1.8 +            continue;
     1.9          else if ( strncmp(p, "com", 3) ||
    1.10                    (sercon_handle = serial_parse_handle(p)) == -1 )
    1.11          {