linux: update help text of BR2_LINUX_KERNEL_INSTALL_TARGET

The BR2_LINUX_KERNEL_INSTALL_TARGET option not only copies the kernel
image to /boot, but also the DTBs, if such DTBs are generated by the
build process. It makes sense to mention that in the help text of this
option.

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