linux port: Update Makefile with PKI=0/1 option
linux port: Update Makefile with PKI=0/1 option

PKI=1 is set to be the default setting and when set builds the stack
with the OC_PKI flag thereby including all support for the certificate
credential type, utilities, PKI APIs, oc_cred and oc_tls code for
handling certificates and sessions employing certificates.

To disable certificate support, we can run "make PKI=0..."

Change-Id: I436d4797e9d56f28d3733247c60c98deba77f9de
Signed-off-by: Kishen Maloor <c62ddea3cd661a9268bff15f07fc2229cdc9ac02@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/27697

iotivity/iotivity-constrained,iotivity/iotivity-constrained,iotivity/iotivity-constrained,iotivity/iotivity-constrained