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

Resolves platformio/platformio-core#3842

Author: valeros

builder/main.py

@@ -248,7 +248,7 @@ def get_esptoolpy_reset_flags(resetmethod):
             join("$BUILD_DIR", "${PROGNAME}"), target_elf)
         env.Depends(target_firm, "checkprogsize")
 
-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)