linux: only depend on host-lzop if needed

There is no reason to always depend on host-lzop, even when the kernel
compression is not LZO.

Since LZO is not the default compression option in the kernel (and there
is not sign that will change in the foreseeable future), it will always
appear in a config file, whether it is a complete config file or it is
only a defconfig.

So, only depend on host-lzop if the LZO compression is enabled in the
kernel config file (either the defconfig or the custom config file).
This includes:
  - kernel compression itself
  - initrd compression
  - initramfs compression

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>