package/linux-headers: really drop the 4.17 choice

Currently, the build fails when the kernel headers 4.17 are selected:

    package/linux-headers/linux-headers.mk:149: *** LINUX_HEADERS_SITE
    cannot be empty when LINUX_HEADERS_SOURCE is not.  Stop.

This is caused by an hickup during the last merge of the next branch,
where the value for 4.17 got dropped from BR2_DEFAULT_KERNEL_HEADERS,
but the choice entry was left in.

The correct fix against that hickup would be to restore the 4.17 value.
But since 4.17 is no longer maintained, the proper fix is to really drop
4.17 altogether.

For good this time! ;-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Config.in.legacy

@@ -200,6 +200,13 @@ config BR2_KERNEL_HEADERS_4_15
 	  Version 4.15.x of the Linux kernel headers are no longer
 	  maintained upstream and are now removed.
 
+config BR2_KERNEL_HEADERS_4_17
+	bool "kernel headers version 4.17.x are no longer supported"
+	select BR2_LEGACY
+	help
+	  Version 4.17.x of the Linux kernel headers are no longer
+	  maintained upstream and are now removed.
+
 ###############################################################################
 comment "Legacy options removed in 2018.08"
 

package/linux-headers/Config.in.host

@@ -56,10 +56,6 @@ config BR2_KERNEL_HEADERS_4_16
 	bool "Linux 4.16.x kernel headers"
 	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_16
 
-config BR2_KERNEL_HEADERS_4_17
-	bool "Linux 4.17.x kernel headers"
-	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_17
-
 config BR2_KERNEL_HEADERS_4_18
 	bool "Linux 4.18.x kernel headers"
 	select BR2_TOOLCHAIN_HEADERS_AT_LEAST_4_18