multiboot2: parse vga= option when setting GOP mode

multiboot2: parse vga= option when setting GOP mode

Introduce support for passing the command line to the efi_multiboot2()
helper, and parse the vga= option if present.

Add support for the 'gfx' and 'current' vga options, ignore the 'keep'
option, and print a warning message about other options not being
currently implemented.

Note that the multboot2 command line string must always be
zero-terminated according to the multiboot2 specification, and hence
there's no need to pass the string size to efi_multiboot2().

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>