arch-wrpc: avoid needless compilation and better error message

The existing rule to check whether WRPCSW_ROOT is properly set (by
environment or default in Kconfig) forced a dependency on all object
file.  Thus, all files were rebuilt every time.

We now force the check only as a dependency for the first object file,
which means ony fsm.o (and then ppsi.o) is built if you run "make" on
a just-built source tree.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>