buildroot

Revert "linux: check the configuration file exists"

This reverts commit b4cacbf5b15db06132b5f401d39b4c8d4e79158e. The
whole host-lzop optional dependency logic cannot work, since the
configuration file will only be known after the kernel sources are
extracted, if an internal kernel defconfig is used, which is quite
common.

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