[FS#1477] toolchain: use external toolchain build error

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - xukaihub (xukaihub) 

Attached to Project - OpenWrt/LEDE Project
Summary - toolchain: use external toolchain build error
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - When I use external toolchain build openwrt

          --- Use external toolchain
          [ ]   Use host's toolchain
          (arm-openwrt-linux-gnueabi) Target name
          (arm-openwrt-linux-gnueabi-) Toolchain prefix
          ($(TOPDIR)/../toolchains/arm) Toolchain root    
          (glibc) Toolchain libc 
          (./usr/bin ./bin) Toolchain program path
          (./usr/include ./include) Toolchain include path
          (./usr/lib ./lib) Toolchain library path

I get build error(external_toolchain.txt log)

make -C /home/xukai/project/commit/robotos/openwrt/build_dir/target-arm-openwrt-linux-gnueabi/linux-leo_gx8010-dev-1v/linux-4.4.25 HOSTCFLAGS="-O2 -I/home/xukai/project/commit/robotos/openwrt/staging_dir/host/include -I/home/xukai/project/commit/robotos/openwrt/staging_dir/host/usr/include  -Wall -Wmissing-prototypes -Wstrict-prototypes" CROSS_COMPILE="arm-openwrt-linux-gnueabi-" ARCH="arm" KBUILD_HAVE_NLS=no KBUILD_BUILD_USER="" KBUILD_BUILD_HOST="" CONFIG_SHELL="bash" V=''  CC="arm-openwrt-linux-gnueabi-gcc" modules
make[5]: Entering directory '/home/xukai/project/commit/robotos/kernel'
  HOSTCC  scripts/kconfig/conf.o
  HOSTCC  scripts/kconfig/zconf.tab.o
  HOSTLD  scripts/kconfig/conf
scripts/kconfig/conf  --silentoldconfig Kconfig
net/sched/Kconfig:44: warning: menuconfig statement without prompt
.config:1092:warning: override: DEFAULT_NOOP changes choice state
#
# configuration written to .config
#
  CHK     include/config/kernel.release
  CHK     include/generated/uapi/linux/version.h
  CHK     include/generated/utsrelease.h
make[6]: 'include/generated/mach-types.h' is up to date.
  CC      kernel/bounds.s
In file included from ./arch/arm/include/asm/bug.h:6:0,
                 from include/linux/bug.h:4,
                 from include/linux/page-flags.h:9,
                 from kernel/bounds.c:9:
./arch/arm/include/asm/opcodes.h:14:52: error: unknown type name 'u32'
 extern asmlinkage unsigned int arm_check_condition(u32 opcode, u32 psr);
                                                    ^
./arch/arm/include/asm/opcodes.h:14:64: error: unknown type name 'u32'
 extern asmlinkage unsigned int arm_check_condition(u32 opcode, u32 psr);
                                                                ^
In file included from include/linux/log2.h:16:0,
                 from kernel/bounds.c:12:
./arch/arm/include/asm/bitops.h: In function '____atomic_set_bit':
include/linux/bitops.h:8:39: error: 'BITS_PER_LONG' undeclared (first use in this function)
 #define BIT_MASK(nr)  (1UL