Application.mk: Add missing preconfig target

When running 'make preconfig' from the apps directory (e.g. to regenerate
Kconfig files with absolute paths), the build fails in subdirectories
that include Application.mk but do not define a preconfig target
(e.g., apps/modbus, apps/nshlib).

This commit adds a dummy preconfig target to Application.mk to ensure
the recursive make successfully traverses these directories.

Author: ThePassionate

Application.mk

@@ -400,3 +400,5 @@ distclean:: clean
 
 # Include Wasm specific definitions
 include $(APPDIR)/tools/Wasm.mk
+
+preconfig: