eSDK.py: unset BBPATH and BUILDDIR to avoid eSDK failure

When executing eSDK test case, the following error appears.

  WARNING: attempting to use the extensible SDK in an environment
  	   set up to run bitbake - this may lead to unexpected
	   results. Please source this script in a new shell session
	   instead.

  FileExistsError: [Errno 17] File exists: '/.../tmp/work/qemux86_64-poky-linux/core-image-minimal/1.0-r0/selftest-esdk-fcuyzsqu/tmp/sysroots/x86_64/bin/pigz' -> '/.../tmp/hosttools/pigz'

So unset these two vars to avoid messing things up.

(From OE-Core rev: bc07d825ce8bb3b337623c232fef61f2781c82af)

(From OE-Core rev: 748f946ee74f7480200a7eb0bb0b695467b08f0a)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/selftest/cases/eSDK.py

@@ -32,7 +32,7 @@ class oeSDKExtSelfTest(OESelftestTestCase):
         if not 'shell' in options:
             options['shell'] = True
 
-        runCmd("cd %s; . %s; %s" % (tmpdir_eSDKQA, env_eSDK, cmd), **options)
+        runCmd("cd %s; unset BBPATH; unset BUILDDIR; . %s; %s" % (tmpdir_eSDKQA, env_eSDK, cmd), **options)
 
     @staticmethod
     def generate_eSDK(image):