build: Add mbedtls to prep script

build: Add mbedtls to prep script

Patch it if patch files present in parent folder, ie:

- extlibs/mbedtls/ocf.patch (md5=96feff0a1627e465e05c350a47464cdf)
  (support for MBEDTLS_TLS_ECDH_ANON_WITH_AES_128_CBC_SHA256)

It is strongly recommended to rely on scons instead,
 if not possible, this can be done differently using this script.

For more details about building IoTivity please check:

https://wiki.iotivity.org/build

Bug: https://jira.iotivity.org/browse/IOT-820
Change-Id: I64a0adbe4977a4138c3d235d247e5e1489220000
Origin: https://gerrit.iotivity.org/gerrit/#/c/11815/
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>