* fix: donot fail if no old package has a build no

* fix: use join

pisi/build.py

@@ -538,8 +538,6 @@ class Builder:
                 a.sort(lambda x,y : cmp(x[1],y[1]))
                 old_package_fn = a[-1][0]   # get the last one
                 old_build = a[-1][1]
-            else:
-                old_build = None
 
                 # compare old files.xml with the new one..
                 old_pkg = Package(old_package_fn, 'r')
@@ -564,6 +562,9 @@ class Builder:
                             if fo.hash != fn.hash:
                                 changed = True
                                 break
+            else: # no old build had a build number
+                old_build = None
+
             ctx.ui.debug('old build number: %s' % old_build)
                             
             # set build number
@@ -650,7 +651,7 @@ class Builder:
             files = Files()
             files.read(ctx.const.files_xml)
             for finfo in files.list:
-                pkg.add_to_package("install/" + finfo.path)
+                pkg.add_to_package(join("install", finfo.path))
 
             pkg.close()
             os.chdir(c)