.xcs/.xcu files can reference other .xcs files via oor:component attributes, and
the various xsltproc scripts called from solenv/gbuild/Configure.mk try to load
the referenced .xcs files, locating them via --stringparam schemaRoot arguments.

These references among .xcs/.xcu files are not tracked via dependencies in the
build system.  When building the primary registry in module officecfg,
schemaRoot must therefore not reference solver, as any referenced .xcs files
need not yet be delivered.

In the old dmake build system, this was solved by basing the schemaRoot argument
on an ABSXCSROOT variable that was set differently in
officecfg/util/makefile.pmk (pointing into officecfg/ itself) than in other
modules (like in connectivity/makefile.pmk, pointing into solver/).

Reproduced that hack in the new gbuild system now, too (which already had that
hack partially implemented for .xcs files).

Change-Id: Ib2300686c4de144f0b1e18d0e985e6fc7d9f64ae