Flush 'Build for ...' messages before each build

Flush 'Build for ...' messages before each build

During a run of 'auto_build.py', stdout buffering get taken over by the
subprocess module, causing print statements in 'auto_build.py' to not
output until the very end of a build. Calling 'sys.stdout.flush()'
causes stdout to be output before the next call to 'subprocess'.

Change-Id: Ibb193b5d30458a3d9be20d418ee22cac302ee819
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>
Reviewed-on: https://gerrit.iotivity.org/gerrit/11823
Reviewed-by: Rick Bell <richard.s.bell@intel.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>