efi_loader: Set default console colors on efi_cout_clear_screen if needed

Ensures a consistent background color of the whole screen for succeeding
outputs as both demanded by the spec and implemented in EDK2 as well.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>