bbconfigbuild/configfragments.py: set BBPATH in standalone datastore

Otherwise fragments with 'require' statements will not parse,
as the parser will not be able to find files referred to by
those statements.

Add such a statement to the test fragment so that the scenario
is tested.

[YOCTO #15707]

(From OE-Core rev: d88e63d1a7867b3a4ebfc57d472900136b5361d8)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-selftest/conf/fragments/test-fragment.conf

@@ -1,3 +1,5 @@
 BB_CONF_FRAGMENT_SUMMARY = "This is a configuration fragment intended for testing in oe-selftest context"
 BB_CONF_FRAGMENT_DESCRIPTION = "It defines a variable that can be checked inside the test."
+
 SELFTEST_FRAGMENT_VARIABLE = "somevalue"
+require conf/multilib.conf

meta/lib/bbconfigbuild/configfragments.py

@@ -23,6 +23,7 @@ def plugin_init(plugins):
 class ConfigFragmentsPlugin(LayerPlugin):
     def get_fragment_info(self, path, name):
         d = bb.data.init()
+        d.setVar('BBPATH', self.tinfoil.config_data.getVar('BBPATH'))
         bb.parse.handle(path, d, True)
         summary = d.getVar('BB_CONF_FRAGMENT_SUMMARY')
         description = d.getVar('BB_CONF_FRAGMENT_DESCRIPTION')