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>

Android.mk

@@ -447,7 +447,7 @@ all_cil_files := \
 $(LOCAL_BUILT_MODULE): PRIVATE_CIL_FILES := $(all_cil_files)
 $(LOCAL_BUILT_MODULE): $(HOST_OUT_EXECUTABLES)/secilc $(HOST_OUT_EXECUTABLES)/sepolicy-analyze $(all_cil_files)
 	@mkdir -p $(dir $@)
-	$(hide) $< -M true -c $(POLICYVERS) $(PRIVATE_CIL_FILES) -o $@.tmp
+	$(hide) $< -M true -c $(POLICYVERS) $(PRIVATE_CIL_FILES) -o $@.tmp -f /dev/null
 	$(hide) $(HOST_OUT_EXECUTABLES)/sepolicy-analyze $@.tmp permissive > $@.permissivedomains
 	$(hide) if [ "$(TARGET_BUILD_VARIANT)" = "user" -a -s $@.permissivedomains ]; then \
 		echo "==========" 1>&2; \
@@ -599,7 +599,7 @@ all_cil_files.recovery := \
 $(LOCAL_BUILT_MODULE): PRIVATE_CIL_FILES := $(all_cil_files.recovery)
 $(LOCAL_BUILT_MODULE): $(HOST_OUT_EXECUTABLES)/secilc $(HOST_OUT_EXECUTABLES)/sepolicy-analyze $(all_cil_files.recovery)
 	@mkdir -p $(dir $@)
-	$(hide) $< -M true -c $(POLICYVERS) $(PRIVATE_CIL_FILES) -o $@.tmp
+	$(hide) $< -M true -c $(POLICYVERS) $(PRIVATE_CIL_FILES) -o $@.tmp -f /dev/null
 	$(hide) $(HOST_OUT_EXECUTABLES)/sepolicy-analyze $@.tmp permissive > $@.permissivedomains
 	$(hide) if [ "$(TARGET_BUILD_VARIANT)" = "user" -a -s $@.permissivedomains ]; then \
 		echo "==========" 1>&2; \