doc: Clarify conditional layer dependencies on meta-arm

The layers from meta-arm are required not only when building for the N1SDP, but
also when building a virtualization image (as bundled Guest VM is generic-arm64
based).

Issue-Id: SCM-4394
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: I23e79bd98b34afa4901535d83f22ce2b129afcf9