build: set OUT_DIR to an absolute path
OUT_DIR was set to $(TOPDIR)out previously, but $(TOPDIR) was null, so it was a relative path.
This broke releasetools, inline kernel building, and many others, since they require absolute paths.
Fix it so that it is set to $(shell readlink -f .)/out if $(TOPDIR) is null.
Also remove hacks which checked if (OUT_DIR) was out and changed it to $(ANDROID_BUILD_TOP)/out to workaround the aforementioned problem.

Conflicts:
    core/base_rules.mk
    core/tasks/kernel.mk