ccache: Allow external setting of CCACHE_COMPILERCHECK variable

This patch allows usage of a custom CCACHE_COMPILERCHECK setting
for the special cases when "content" strategy is not enough to ensure
that results retrieved from the cache are accurate for the current
compiler.

BUG=none
Change-Id: I30be5182cfb36ec30f38b9a2fac89290bd5df910
Signed-off-by: Stefan Stanacar <stefan.stanacar@intel.com>
diff --git a/core/ccache.mk b/core/ccache.mk
index 5c2ae23..d67bce6 100644
--- a/core/ccache.mk
+++ b/core/ccache.mk
@@ -17,7 +17,7 @@
 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
+  export CCACHE_COMPILERCHECK ?= content
 
   # See man page, optimizations to get more cache hits
   # implies that __DATE__ and __TIME__ are not critical for functionality.