linux: fix the KCONFIG_GET_OPT calls to be inside a BR2_LINUX_KERNEL test

The KCONFIG_GET_OPT calls added by
ca80782f ('linux: only depend on
host-lzop if needed') are made even if the kernel package is not
selected. This hangs the linux.mk parsing as they try to read from a
file that doesn't exist.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>