qemuboot: don't fail when QB_DEFAULT_KERNEL isn't symlink

qemuboot: don't fail when QB_DEFAULT_KERNEL isn't symlink

* in some cases we might set QB_DEFAULT_KERNEL to the real filename
  instead of symlink and then this whole readlink work around actually
  breaks the build, because os.readlink fails on normal files:

  >>> os.readlink('deploy/images/qemux86/bzImage-linux-yocto-qemux86-master-20160927084848.bin')
  'bzImage-linux-yocto-qemux86.bin'
  >>> os.readlink('deploy/images/qemux86/bzImage-linux-yocto-qemux86.bin')
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  OSError: [Errno 22] Invalid argument: '/jenkins/mjansa/build-starfish-master-mcf/BUILD/deploy/images/qemux86/bzImage-linux-yocto-qemux86.bin'

(From OE-Core rev: a11d0d8641b7dfb05c78645cf21f2c04a08c4822)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>