efi_loader: rework bootmgr/bootefi using load_image API

In the current implementation, bootefi command and EFI boot manager
don't use load_image API, instead, use more primitive and internal
functions. This will introduce duplicated code and potentially
unknown bugs as well as inconsistent behaviours.

With this patch, do_efibootmgr() and do_boot_efi() are completely
overhauled and re-implemented using load_image API.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>

Use efi_root as parent handle for the loaded image.
LoadImage() should be called with BootPolicy = true by the boot manager.
Avoid duplicate free_pool().
Eliminate variable memdp which is not needed after anymore due to
"efi_loader: correctly split device path of loaded image".

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>