sstate.bbclass: Ensure build directory is cleaned to start with

This directory is cleaned upon completion however if a previous build
crashes, it can lead to corrpution, hence ensure its clean at the start
too.

(From OE-Core rev: 8ef0e59d5a7da3671d1ad9a54fe068ed78f928d5)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/sstate.bbclass

@@ -444,6 +444,7 @@ def sstate_package(ss, d):
 
     sstatebuild = d.expand("${WORKDIR}/sstate-build-%s/" % ss['name'])
     sstatepkg = d.getVar('SSTATE_PKG', True) + '_'+ ss['name'] + ".tgz"
+    bb.utils.remove(sstatebuild, recurse=True)
     bb.mkdirhier(sstatebuild)
     bb.mkdirhier(os.path.dirname(sstatepkg))
     for state in ss['dirs']: