sstate: Ensure installation directory is empty before execution

When you enable the systemd DISTRO_FEATURE, opkg-native contains systemd units
which have a relocation fixme list. When systemd isn't in DISTRO_FEATURES, there
are no fixmes required. Unfortunately as sstate isn't cleaning up its installation
directory before use, if you install the systemd version, then install the
non-systemd version from sstate, it would leave behind the fixme file from the
systemd version and breakage results as it would try and fixup files which don't
exist.

The solution is to ensure the unpack/install directory is clean before use. It
does raise other questions about opkg-native, systemd and DISTRO_FEATURES but there
is an underlying sstate issue here too which would cause missing file failures.

(From OE-Core rev: d1d55041e38b12d40f896834b56475ea19a6047f)

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

meta/classes/sstate.bbclass

@@ -313,6 +313,11 @@ def sstate_installpkg(ss, d):
         if not signer.verify(sstatepkg + '.sig'):
             bb.warn("Cannot verify signature on sstate package %s" % sstatepkg)
 
+    # Empty sstateinst directory, ensure its clean
+    if os.path.exists(sstateinst):
+        oe.path.remove(sstateinst)
+    bb.utils.mkdirhier(sstateinst)
+
     sstateinst = d.getVar("SSTATE_INSTDIR")
     d.setVar('SSTATE_FIXMEDIR', ss['fixmedir'])