Bump version to v1.6.0

We've accumulated some new features and a bunch of fixes.  Also the
versioning on v1.5.1 was messed up :(.  Prepare for another release.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Makefile

@@ -10,7 +10,7 @@
 # CONFIG_LOCALVERSION from some future config system.
 #
 VERSION = 1
-PATCHLEVEL = 5
+PATCHLEVEL = 6
 SUBLEVEL = 0
 EXTRAVERSION =
 LOCAL_VERSION =