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

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.

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