bitbake.conf: Add LICENSE_PATH to sstate whitelist

some layers e.g. meta-intel are defining LICENSE_PATH in layer.conf like
LICENSE_PATH += "${LAYERDIR}/common/custom-licenses"
This causes the target packages to invalidate the siginfo and casues
rebuild for all packages.

Thanks bitbake-diffsigs, it shows

basehash changed from c27b55ea6980262fab370e539ba8bcd7 to
37962b22ebd6194e9537bc4f85819323
Variable LICENSE_PATH value changed from '
/builds2/poky/meta-intel/common/custom-licenses' to '
/b/kraj/jlinux-next/poky/meta-intel/common/custom-licenses'

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>