Fix image rules wrt system/ relative entries

* Introduce on-container jam variable HAIKU_CONTAINER_SYSTEM_DIR_TOKENS
  containing the directory tokens relative to the container root to
  refer to system.
* Use the variable in *ToContainer rules that need to put something in
  system.
* AddFilesToContainer: Fix overlooked reference to AddFilesToHaikuImage.
* AddBootModuleSymlinksToContainer: Use relative symlinks. This avoids
  special-casing in the boot loader for packagefs.

build/jam/PackageRules

@@ -272,6 +272,7 @@ rule HaikuPackage package
 		= $(grist)_HAIKU_INCLUDE_IN_PACKAGE ;
 	HAIKU_INSTALL_TARGETS_VAR on $(package)
 		= $(grist)_HAIKU_PACKAGE_INSTALL_TARGETS ;
+	HAIKU_CONTAINER_SYSTEM_DIR_TOKENS on $(package) = ;
 
 	HAIKU_CURRENTLY_BUILT_HAIKU_PACKAGE = $(package) ;
 }