meta/PRService: Added export/import fuctions.

[YOCTO #1556]
- Modified meta/class/package.bbclass and prserv.bbclass according to
the change in PR service by adding PACKAGE_ARCH into the query tuple.

- Added prexport.bbclass, primport.bbclass to export/import AUTOPR
  values from/to PRService.

- Move PR service related common code to lib/oe/prservice.py.

- Supported reading the AUTOPR values from the exported .inc file
instead of reading it from remote PR service.

- Created a new script bitbake-prserv-tool to export/import the AUTOPR
values from/to the PR service.

Typical usage scenario of the export/import is:
1. bitbake-prserv-tool export <file> to export the AUTOPR values from
the current PR service into an exported .inc file.

2. Others may use that exported .inc file(to be included in the
local.conf) to lockdown and reproduce the same AUTOPR when generating
package feeds.

3. Others may "bitbake-prserv-tool import <file>" to import the AUTOPR
values into their own PR service and the AUTOPR values will be
incremented from there.

(From OE-Core rev: 9979107d8eaf503efd921564385859b1e83dbb3c)

Signed-off-by: Lianhao Lu <lianhao.lu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/package.bbclass

@@ -351,10 +351,17 @@ def runtime_mapping_rename (varname, d):
 
 python package_get_auto_pr() {
 	if d.getVar('USE_PR_SERV', True) != "0":
-		auto_pr=prserv_get_pr_auto(d)
-		if auto_pr is None:
-			bb.fatal("Can NOT get auto PR revision from remote PR service")
+		try:
+			auto_pr=prserv_get_pr_auto(d)
+		except Exception as e:
+			bb.fatal("Can NOT get PRAUTO, exception %s" %  str(e))
 			return
+		if auto_pr is None:
+			if d.getVar('PRSERV_LOCKDOWN', True):
+				bb.fatal("Can NOT get PRAUTO from lockdown exported file")
+			else:
+				bb.fatal("Can NOT get PRAUTO from remote PR service")
+			return 
 		d.setVar('PRAUTO',str(auto_pr))
 }