Revert "make partial_build work in separate builddir"

This reverts commit 207e370a.

This breaks the module build i.e. cd <module> && make.

solenv/gbuild/partial_build.mk

 ifeq ($(gb_Side),)
 gb_Side:=host
 endif
-
-ifeq (,$(BUILDDIR))
-BUILDDIR := $(dir $(realpath $(firstword $(MAKEFILE_LIST))))../..
-endif
-
 ifeq ($(SOLARENV),)
-include $(BUILDDIR)/config_$(gb_Side).mk
+include $(dir $(realpath $(lastword $(MAKEFILE_LIST))))../../config_$(gb_Side).mk
 endif
 
 gb_PARTIAL_BUILD := T