configs/clicker2-stm32: Fixes minor SPI configuration issues

Approved-by: Gregory Nutt <gnutt@nuttx.org>

realloc(): When realloc() has to fall back to calling malloc(), size including overhead was being provided to malloc(), causing a slightly larger allocation than needed.  Noted by initialkjc@yahoo.com

pthread mutexes:  Add option to support both unsafe and robust mutexes via pthread_mutexattr_get/setrobust().

pthreads:  Add a configuration option to disable robust mutexes and revert to the traditional unsafe mutexes.