Disable goma/ccache with USE_XXX=false

Bug: 26009364
Change-Id: Ic0001e6fdc62ca2373309bfe3127c5fec64d2d85
diff --git a/core/ccache.mk b/core/ccache.mk
index c99d933..5c2ae23 100644
--- a/core/ccache.mk
+++ b/core/ccache.mk
@@ -14,7 +14,7 @@
 # limitations under the License.
 #
 
-ifneq ($(USE_CCACHE),)
+ifneq ($(filter-out false,$(USE_CCACHE)),)
   # The default check uses size and modification time, causing false misses
   # since the mtime depends when the repo was checked out
   export CCACHE_COMPILERCHECK := content