oe/copy_buildsystem: check_sstate_task_list also pop BBPATH from env

The BBPATH environment could be set and can make a failure when try
to build an extensible sdk because it will look the bitbake.lock
file in the original build folder.

Example:

$ export BBPATH=`pwd`
$ bitbake core-image-minimal -c populate_sdk_ext

ERROR: bitbake failed:
ERROR: Only one copy of bitbake should be run against a build directory
ERROR: core-image-minimal-1.0-r0 do_populate_sdk_ext: Function failed:
copy_buildsystem

(From OE-Core rev: 33634b4c38d84e1c5d06056766933f1fe4f47e8d)

Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oe/copy_buildsystem.py

@@ -239,6 +239,7 @@ def check_sstate_task_list(d, targets, filteroutfile, cmdprefix='', cwd=None, lo
     cmd = "%sBB_SETSCENE_ENFORCE=1 PSEUDO_DISABLED=1 oe-check-sstate %s -s -o %s %s" % (cmdprefix, targets, filteroutfile, logparam)
     env = dict(d.getVar('BB_ORIGENV', False))
     env.pop('BUILDDIR', '')
+    env.pop('BBPATH', '')
     pathitems = env['PATH'].split(':')
     env['PATH'] = ':'.join([item for item in pathitems if not item.endswith('/bitbake/bin')])
     bb.process.run(cmd, stderr=subprocess.STDOUT, env=env, cwd=cwd, executable='/bin/bash')