cmd: efidebug: add "test bootmgr" sub-command

This sub-command will be used to test image authentication,
in particular, a case where efi_load_image() failed with
EFI_SECURITY_VIOLATION but we still want to try efi_start_image().
We won't run such a case under normal bootmgr because it simply
refuses to call efi_start_image() if anything but EFI_SUCCESS
is returned when loading an image.

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