gen-lockedsig-cache: fix bad destination path joining

When copying the sstate-cache into the extensible SDK, if the source
path had a trailing / and the destination path did not, there would be a
missing / between the path and the subdirectory name, and you'd end up
with subdirectories like "sstate-cacheCentOS-6.7". There are functions
in os.path for this sort of thing so let's just use them and avoid the
problem.

(From OE-Core rev: 5eb8f15c48b5f39a10eb2b63b026cf1ebfd05533)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/gen-lockedsig-cache

@@ -41,7 +41,7 @@ for f in files:
         # Most likely a temp file, skip it
         print('skipping')
         continue
-    dst = f.replace(sys.argv[2], sys.argv[3])
+    dst = os.path.join(sys.argv[3], os.path.relpath(f, sys.argv[2]))
     destdir = os.path.dirname(dst)
     mkdir(destdir)