build: stop generating $T/file_contexts

secilc is being used without -f which is causing a file_contexts
file to be generated in the root of the tree where the build tools
run:

$ stat $T/file_contexts
  File: 'file_contexts'
  Size: 0         	Blocks: 0          IO Block: 4096   regular empty file
Device: fc00h/64512d	Inode: 5508958     Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/wcrobert)   Gid: ( 1000/wcrobert)
Access: 2017-03-23 11:23:41.691538047 -0700
Modify: 2017-03-23 11:23:41.691538047 -0700
Change: 2017-03-23 11:23:41.691538047 -0700

Test: remove $T/file_contexts, touch a policy file and make sepolicy,
      ensure file is not regenerated. Also, ensure hikey builds and
      boots.

Change-Id: I0d15338a540dba0194c65a1436647c7d38fe3c79
Signed-off-by: William Roberts <william.c.roberts@intel.com>