efi_loader: console: Correctly report modes

Add support for EFI console modes.
Mode 0 is always 80x25 and present by EFI specification.
Mode 1 is always 80x50 and not mandatory.
Mode 2 and above is freely usable.

If the terminal can handle mode 1, we mark it as supported.
If the terminal size is greater than mode 0 and different than mode 1,
we install it as mode 2.

Modes can be switch with cout_set_mode.

Changes in V5:
 Correctly detect mode before enabling mode 2.

Changes in V4:
 Reset cursor positon on mode switch
 Use local variables in console query code

Changes in V3:
 Valid mode are 0 to EFIMode-1
 Fix style

Changes in V2:
 Add mode switch
 Report only the modes that we support

Signed-off-by: Emmanuel Vadot <manu@bidouilliste.com>
Signed-off-by: Alexander Graf <agraf@suse.de>