Makefile.am - comment why control.rng is in EXTRA_DIST
Makefile.am - comment why control.rng is in EXTRA_DIST

- additionaly agregate all XML files and call `xmllint` just once

jsrain/skelcd-control-SLES,sndirsch/skelcd-control-openSUSE,bmwiedemann/skelcd-control-openSUSE,lnussel/skelcd-control-openSUSE,yast/skelcd-control-SLES,yast/skelcd-control-openSUSE,bmwiedemann/skelcd-control-openSUSE,kobliha/skelcd-control-openSUSE,yast/skelcd-control-SLED,sndirsch/skelcd-control-openSUSE,lnussel/skelcd-control-openSUSE,yast/skelcd-control-SLED,yast/skelcd-control-SLES,yast/skelcd-control-openSUSE,jsrain/skelcd-control-SLES,dirkmueller/skelcd-control-openSUSE,dirkmueller/skelcd-control-openSUSE,kobliha/skelcd-control-openSUSE