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>

board/tcm-bf518/config.mk

@@ -25,3 +25,4 @@
 
 CFLAGS_lib += -O2
 CFLAGS_lib/lzma += -O2
+CFLAGS_lib/zlib += -O2

board/tcm-bf537/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 --dma 8