Check first if system_dir and device_dir exists

Change the order of variable checking so it is clear whether
the system_dir or device_dir does not exist or a specific file
in the *_dir does not exist.

Test: build_mixed system_dir device_dir dist_dir
Merged-In: I0d61ea11c3bd22ef12eccbb46b8f665414188307
Change-Id: I0d61ea11c3bd22ef12eccbb46b8f665414188307
(cherry picked from commit 52edf8fb)