core/Makefile: Define write-file-lines unconditionally

Right now write-file-lines is defined only if BUILDING_SYSTEM_IMAGE is
true. This means split build / partial build targets that doesn't build
the system image, would fail to build other partition images too
because the $(<parttition>image_intermediates)/file_list.txt build rule
would be undefined.

Move the macro definition out of the `ifdef BUILDING_SYSTEM_IMAGE`
conditional block.

Bug: 326359088
Test: build a target that doesn't build system image
Change-Id: I020c28f7011bd0df0330ba3350e50bea2b75aab4