Properly declare buildfs target so it can be used for pre/post actions

Author: valeros

builder/main.py

@@ -232,7 +232,7 @@ def __fetch_spiffs_size(target, source, env):
         target_firm = env.ElfToBin(
             join("$BUILD_DIR", "${PROGNAME}"), target_elf)
 
-env.AddPlatformTarget("buildfs", target_firm, None, "Build Filesystem Image")
+env.AddPlatformTarget("buildfs", target_firm, target_firm, "Build Filesystem Image")
 AlwaysBuild(env.Alias("nobuild", target_firm))
 target_buildprog = env.Alias("buildprog", target_firm, target_firm)