linux.inc: support building with B != S which is default now

* this .inc file wasn't working since oe-core commit:
  commit 106dab2fd0321e6b4e77b40111e59a3a31d329d4
  Author: Richard Purdie <richard.purdie@linuxfoundation.org>
  Date:   Fri Dec 19 17:46:27 2014 +0000
  Subject: kernel: Fix non linux-yocto builds
* Fixes following do_configure error
  | NOTE: make oldconfig
  | make: *** No rule to make target `oldconfig'.  Stop.
  | ERROR: oe_runmake failed

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>