[cmake-build] ensure cc2538 uses 'CMAKE_BUILD_TYPE=Release' (#5669)
[cmake-build] ensure cc2538 uses 'CMAKE_BUILD_TYPE=Release' (#5669)

This change ensures the cc2538 cmake build uses the release build type
and the compile option `-Os` (optimize for size). This harmonizes the
cmake build with automake version. It also helps address an issue with
recent PRs failing on cc2538 cmake build with arm-gcc-5 due to
reaching flash size limit (without `-Os` option).
srickardti/openthread,srickardti/openthread,chshu/openthread,srickardti/openthread,openthread/openthread,openthread/openthread,jwhui/openthread,bukepo/openthread,abtink/openthread,openthread/openthread,abtink/openthread,chshu/openthread,jwhui/openthread,chshu/openthread,chshu/openthread,jwhui/openthread,abtink/openthread,chshu/openthread,jwhui/openthread,chshu/openthread,bukepo/openthread,bukepo/openthread,openthread/openthread,bukepo/openthread,srickardti/openthread,abtink/openthread