Build cache.img on demand

Bug: 5153694
To build cache.img, set BOARD_CACHEIMAGE_FILE_SYSTEM_TYPE (required,
ext4 only for now), BOARD_CACHEIMAGE_PARTITION_SIZE (optional) in
BoardConfig.mk.

Change-Id: I1d8b91646aa1dba88285e008ad3335768bcbddd2
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 4957354..1049591 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -138,6 +138,7 @@
   makeint("blocksize")
   makeint("system_size")
   makeint("userdata_size")
+  makeint("cache_size")
   makeint("recovery_size")
   makeint("boot_size")