Refine makefiles

[domna]

This adds several things:

• Proper build of nxdl.xml files even if they are not present. This is now completely based on the available yaml files
• Removal of nyaml for loop
• Add a new Makefile nyaml.mk for building nyaml targets to be invoked like this make -f nyaml.mk

[domna]

@sanbrock Everything fixed and should work with make nxdl and make nyaml respectively. This also only builds for updated timestamps and based on the existing yaml and nxdl files. Especially, now new yaml files are also converted to nxdl which was not the case previously

[domna]

@sanbrock I aligned everything with nexusformat#1336

[sanbrock]

LGTM