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>

linux/Config.in

@@ -297,6 +297,9 @@ config BR2_LINUX_KERNEL_INSTALL_TARGET
 	  /boot in the target root filesystem, as is typically done on
 	  x86/x86_64 systems.
 
+	  Note that this option also installs the Device Tree Blobs to
+	  /boot if DTBs have been generated by the kernel build
+	  process.
 
 # Linux extensions
 source "linux/Config.ext.in"