yocto-bsp: Add missing format specifier in bblayers error message

If the build environment is misconfigured (e.g. a bad path
for a layer in bblayers.conf) the yocto-bsp script crashes with a
standard python error, not very explicit.  This fixes the problem.

 Signed-off-by: Bastien JAUNY <bastien.jauny@gmail.com>

(From meta-yocto rev: 4a8e80b812eebdc1c9570b5d88aa0f3b34824b68)

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

scripts/lib/bsp/kernel.py

@@ -62,7 +62,7 @@ def find_bblayers():
             break
 
     if not bblayers:
-        print "Couldn't find BBLAYERS in 'bitbake -e' output, exiting." % \
+        print "Couldn't find BBLAYERS in %s output, exiting." % \
             bitbake_env_cmd
         sys.exit(1)