Show error on Unknown Make target

Created by: Dietr1ch

Problem: Make fails when submodules are missing

Solution: Suggests to get submodules on unknown target

Changes introduced by this pull request:

• Add a warning on a default target

Drawbacks: Make still shows the missing target, so it has no apparent drawbacks.

TODOs: -

Fixes: -

State: Ready

Related: #642 (closed)