Fix: mbedtls build error if using prefetched src

mbedtls external component build fails with error:
   "Configuration step of Mbed TLS failed with No such file or
    directory."
when MBEDTLS_SOURCE_DIR is set to use prefetched source-code.
The error is caused by execute_process() being called with incorrect
parameters making the command depend on the MBEDTLS_BINARY_DIR
existing when the call is made.
Change execute_process() to call cmake with the -B option to set the
build directory instead of relying on the working directory being
correctly set. This way the directory will be created when needed.

Change-Id: Ia2b5fee36e160d381fbf45793152efe2fc873210
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>
1 file changed