build: fix kernel_*config targets on 4.19

Recent kernel versions need toolchain in order to properly configure
kernel, for example to set CC_HAS_ASM_GOTO config symbol, so missing
toolchain leads to the following error while running `make
kernel_oldconfig` on ath79 target with 4.19 kernel in the clean tree:

 init/Kconfig:17: syntax error
 init/Kconfig:16: invalid option
 ./scripts/clang-version.sh: line 15: mips-openwrt-linux-musl-gcc: command not found
 ./scripts/gcc-plugin.sh: line 11: mips-openwrt-linux-musl-gcc: command not found
 net/sched/Kconfig:44: warning: menuconfig statement without prompt
 make[4]: *** [scripts/kconfig/Makefile:69: oldconfig] Error 1

So in order to support this new kernel versions, this patch adds
toolchain as a hard dependency to the kernel_*config targets.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

include/toplevel.mk

@@ -145,7 +145,7 @@ xconfig: scripts/config/qconf prepare-tmpinfo FORCE
 	fi
 	$< Config.in
 
-prepare_kernel_conf: .config FORCE
+prepare_kernel_conf: .config toolchain/install FORCE
 
 ifeq ($(wildcard staging_dir/host/bin/quilt),)
   prepare_kernel_conf: