efi_loader: signature of StartImage and Exit

We use u16* for Unicode strings and efi_uintn_t for UINTN. Correct the
signature of efi_exit() and efi_start_image().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>