populate_sdk_ext: Add support for PR service

In the classes/populate_sdk_ext.bbclass the system already copies a number
of configurations, such as the hash equivalency data.  However, the PR
service was being handled.

The new code works by checking if PRSERV_HOST is defined, if it is, use
the existing export functions to write out a conf/prserv.inc file into
the eSDK.  On eSDK install, if a conf/prserv.inc file is present we then
import this file into the system.

This mechanism will work if the PRSERV_HOST is local or remote, as it pulls
the necessary data from the server and then imports it to a local database
on eSDK installation.

Note: the conf/prserv.inc file is not deleted at this time.  It was left
for possible debugging purposes, but removing it is something we could decide
to do in the future.

Signed-off-by: Mark Hatle <mark.hatle@xilinx.com>
Signed-off-by: Mark Hatle <mark.hatle@kernel.crashing.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>