changetarget - allow change of rpmbuilds --target through a macro in prjconf.

changetarget - allow change of rpmbuilds --target through a macro in prjconf.
For building ARM packages, we need to be able to set the target of the build because in the qemu-usermode environment
this is not what rpmbuild autodetects. With this patch we can use this in the prjconf:
%ifarch armv5el
Changetarget: armv5tel-suse-linux
%endif
'osc build' needs this change, too. Otherwise we'd end up with a wrong autodetection when doing a osc build on real hardware > armv5.