linux: clarify help text for 'custom tarball'

The help text for Linux option 'Custom tarball' only refers to ftp or
http tarballs, while in reality file or scp protocols are also
supported.

Triggered by a recent support question, update the help text to clarify
this.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

linux/Config.in

@@ -48,8 +48,13 @@ config BR2_LINUX_KERNEL_CUSTOM_VERSION
 config BR2_LINUX_KERNEL_CUSTOM_TARBALL
 	bool "Custom tarball"
 	help
-	  This option allows to specify the http or ftp location of a
-	  specific kernel source tarball
+	  This option allows to specify a URL pointing to a kernel source
+	  tarball. This URL can use any protocol recognized by Buildroot,
+	  like http://, ftp://, file:// or scp://.
+
+	  When pointing to a local tarball using file://, you may want to
+	  use a make variable like $(TOPDIR) to reference the root of the
+	  Buildroot tree.
 
 config BR2_LINUX_KERNEL_CUSTOM_GIT
 	bool "Custom Git repository"