dont overrides mbedtls's CFLAGS

From mbedtls README.md, "Please note that setting CFLAGS overrides its default value of -O2 and...".
Restore the -O2 will save ~100k for any of mbedtls's lib files.

README.md

@@ -284,7 +284,7 @@ export MBEDTLS_VER=2.6.0
 wget https://tls.mbed.org/download/mbedtls-$MBEDTLS_VER-gpl.tgz
 tar xvf mbedtls-$MBEDTLS_VER-gpl.tgz
 pushd mbedtls-$MBEDTLS_VER
-make SHARED=1 CFLAGS=-fPIC
+make SHARED=1 CFLAGS="-O2 -fPIC"
 sudo make DESTDIR=/usr install
 popd
 sudo ldconfig