mbedtls: Cosmetic cleanups

mbedtls: Cosmetic cleanups

This is more of a cosmetic change and a reminder that the CMake script hardcodes -O2.
Source:
https://github.com/ARMmbed/mbedtls/blob/mbedtls-2.7/CMakeLists.txt#L73
https://github.com/ARMmbed/mbedtls/blob/master/CMakeLists.txt#L97

Remove the release type option as it's already provided by the toolchain.
Source:
https://github.com/openwrt/openwrt/blob/master/include/cmake.mk#L50

Signed-off-by: Daniel Engberg <daniel.engberg.lists@pyret.net>
(backported from 5297a759aee34952299d1d42f677f31781026c67)