boot: Fix computation of TOP

Test Plan: Run `./boot`, check to make sure that
`libraries/ghc-prim/GNUmakefile` is sane

Subscribers: thomie, carter, sjakobi

Differential Revision: https://phabricator.haskell.org/D4580

boot

@@ -116,7 +116,7 @@ def boot_pkgs():
             if os.path.isfile(cabal):
                 # strip both .cabal and .in
                 pkg = os.path.splitext(os.path.splitext(os.path.basename(cabal))[0])[0]
-                top = package
+                top = os.path.join(*['..'] * len(os.path.normpath(package).split(os.path.sep)))
 
                 ghc_mk = os.path.join(package, 'ghc.mk')
                 print('Creating %s' % ghc_mk)