bootefi: allow return without EFI_BOOT_SERVICES.Exit

bootefi: allow return without EFI_BOOT_SERVICES.Exit

The Unified Extensible Firmware Interface Specification, version 2.7,
defines in chapter 2.1.2 - UEFI Application that an EFI application may
either directly return or call EFI_BOOT_SERVICES.Exit().

Unfortunately U-Boot makes the incorrect assumption that
EFI_BOOT_SERVICES.Exit() is always called.

So the following application leads to a memory exception on the aarch64
architecture when returning:

EFI_STATUS efi_main(
  EFI_HANDLE handle,
  EFI_SYSTEM_TABlE systable) {
	return EFI_SUCCESS;
}

With this patch the entry point is stored in the image handle.

The new wrapper function do_enter is used to call the EFI entry point.

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