package/pkg-kernel-module: disable CONFIG_TRIM_UNUSED_KSYMS

The kernel option CONFIG_TRIM_UNUSED_KSYMS=y allows to trim unused
kernel symbols... which basically prevents building external
modules. This option has been enabled in the default MIPS architecture
kernel defconfig since pretty much forever, and it causes build
failures of all our external modules on MIPS in the autobuilders.

To fix this in a global manner, we handle the problem in the
kernel-module infrastructure, by appending to the
<pkg>_LINUX_CONFIG_FIXUPS variable provided by the package.

Fixes:

  http://autobuild.buildroot.net/results/c26abec4e24d06557079654b12825efb2211996f/ (cryptodev-linux)
  http://autobuild.buildroot.net/results/699c618730cb33f0ccf54f90e4b722198fd3d3bc/ (emlog)
  http://autobuild.buildroot.net/results/f1d5626f5698a955bf378217528fbf4f8f741435/ (rtl8812au-aircrack-ng)
  http://autobuild.buildroot.net/results/c9a507757ce3a01cd08d596cb519ee9f19caf77c/ (mdio-tools)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>
(cherry picked from commit 1d8d9e8d6cd824b2a43a0b92d71cfb25a3f9300a)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/pkg-kernel-module.mk

@@ -62,6 +62,9 @@ $(2)_MAKE ?= $$(BR2_MAKE)
 # the root of the package.
 $(2)_MODULE_SUBDIRS ?= .
 
+$(2)_LINUX_CONFIG_FIXUPS += \
+	$$(sep)$$(call KCONFIG_DISABLE_OPT,CONFIG_TRIM_UNUSED_KSYMS)
+
 # Build the kernel module(s)
 # Force PWD for those packages that want to use it to find their
 # includes and other support files (Booo!)