Makefile: hack out the double version of SUBLEVEL

Makefile: hack out the double version of SUBLEVEL

Portage's linux-info.eclass (in getfilevar_noexec) looks for the
definition of SUBLEVEL, and it doesn't expect 2 definitions. We could
fix the eclass, but let's hack this out for now.

See strongswan's emerge output:

...
 * Found sources for kernel version:
 *     4.4.6
 * 0
/mnt/host/source/src/third_party/portage-stable/eclass/linux-info.eclass: line 388: 6
0: syntax error in expression (error token is "0")
...

Change-Id: I6964e6731ed461ca3a8c4afde0ddfe48e0105627
Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-on: https://chrome-internal-review.googlesource.com/252620
Reviewed-by: Doug Anderson <dianders@google.com>
Tested-by: Doug Anderson <dianders@google.com>