aedi: add build prefix target

aedi/target/__init__.py

@@ -118,6 +118,7 @@ def targets():
         ZipTarget(),
 
         # Special
+        BuildPrefix(),
         CleanAllTarget(),
         CleanDepsTarget(),
         DownloadCMakeTarget(),

aedi/target/special.py

@@ -24,6 +24,11 @@ from ..state import BuildState
 from .base import BuildTarget, Target
 
 
+class BuildPrefix(Target):
+    def __init__(self, name='build-prefix'):
+        super().__init__(name)
+
+
 class CleanTarget(Target):
     def __init__(self, name=None):
         super().__init__(name)