linux: clarify the BR2_LINUX_KERNEL_DEFCONFIG help text

The help text of BR2_LINUX_KERNEL_DEFCONFIG is confusing: it seems to
point to the defconfigs in the buildroot tree.  So explicitly refer to
the kernel tree in the help text.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>