envsetup: set OUT_DIR to an absolute path always

OUT_DIR was set to $(TOPDIR)out previously,
but $(TOPDIR) was null, so it was a relative path.
This broke releasetools, inline kernel building, etc
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.

Change-Id: I459a3b1325a1bbea0565cd73f6acf160d4ed9b39

Conflicts:

	core/base_rules.mk
fix sdk target with absolute OUT_DIR

Add / to atree's search path.

since I459a3b13 OUT_DIR is absolute. atree uses these variables and
due to "-I ." tries to resolve these paths as relative paths.

Change-Id: Ic9fbc84acdd9f438dfc813f6496e2c5e0d761976