Removed `config' target, as it only has $(CONFIG_HEADER) dependency.
$(VERSION_HEADER) dependency is in top-make for CRIU's files:
>  criu/%: images/built-in.o compel/compel $(VERSION_HEADER) .FORCE
Introduced pattern-specific target for CRIU files, so it
got as dependency pie (and config header as pie's deps).
As we're building from NMK and including source Makefile from there,
it adds Makefile dependency - tell GNU/Make, that it's all right,
up-to-date by introducing Makefile target (otherwise, Makefile
would fit pattern-specific rule).
For any PIE's object I just rebuild/regenerate all pie, it may
be smarter - but we have now Makefile.library and Makefile
for PIE, so need to know, who should rebuild it.

Fixes:
[criu]$ make criu/log.o
make[1]: Entering directory '/home/japdoll/tools/criu'
make[1]: Leaving directory '/home/japdoll/tools/criu'
make[1]: Entering directory '/home/japdoll/tools/criu'
make[1]: 'images/built-in.o' is up to date.
make[1]: Leaving directory '/home/japdoll/tools/criu'
make[1]: Entering directory '/home/japdoll/tools/criu'
make[1]: 'compel/compel' is up to date.
make[1]: Leaving directory '/home/japdoll/tools/criu'
make[1]: Entering directory '/home/japdoll/tools/criu'
  DEP      criu/log.d
In file included from criu/log.c:22:0:
/home/japdoll/tools/criu/criu/include/string.h:11:20: fatal error: config.h: No such file or directory
 #include "config.h"
                    ^
compilation terminated.
  CC       criu/log.o
In file included from criu/log.c:22:0:
/home/japdoll/tools/criu/criu/include/string.h:11:20: fatal error: config.h: No such file or directory
 #include "config.h"
                    ^
compilation terminated.
make[1]: *** [/home/japdoll/tools/criu/scripts/nmk/scripts/build.mk:104: criu/log.o] Error 1
make[1]: Leaving directory '/home/japdoll/tools/criu'
make: *** [Makefile:200: criu/log.o] Error 2

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>