Blackfin: boards: build zlib dir with -O2

Now that the zlib code has been relocated to a dedicated subdir, make
sure we still build it with -O2 for boards that want speed over size.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/board/bf561-ezkit/config.mk b/board/bf561-ezkit/config.mk
index 4c811ba..4d6184e 100644
--- a/board/bf561-ezkit/config.mk
+++ b/board/bf561-ezkit/config.mk
@@ -25,6 +25,7 @@
 
 CFLAGS_lib += -O2
 CFLAGS_lib/lzma += -O2
+CFLAGS_lib/zlib += -O2
 
 # Set some default LDR flags based on boot mode.
 LDR_FLAGS-BFIN_BOOT_PARA := --bits 16