Revert "Revert "make partial_build work in separate builddir""

This reverts commit 13034c8e.  The original
presumably works fine for a separate builddir(?), but it erroneously(?) replaced
"lastword MAKEFILE_LIST" with "firstword MAKEFILE_LIST" so "cd <module> &&
make" failed to find config_host.mk; fixed that part now.

Change-Id: Icde8315425dd230544125901de4f3a605b239155

solenv/gbuild/partial_build.mk

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