add a custom toolchain for customized use (#8077)

* add a linux_arm toolchain for customized use

This tool chain is targeted to Linux running on ARM SoC, because there
is a demand of running some of the example tools like "chip-tool" on
embeded linux device instead of on Linux laptop.

If the target_os is linux and the target_cpu is "arm" or "arm64", this
toolchain will be used.

It is similar to the "CROSS_COMPILE" when build kernel, a build arg
named "linux_arm_cross_compile" need to be specified also, it should be
a path to the cross compile tools.

Take the embedded linux SDK into consideration, other build args to
specify in the command line includes:
  * target_os
  * target_cpu
  * arm_arch
  * target_cflags
  * target_ldflags
  * etc.

And the pkg-config tool need below two enviroment to be well set  to
correctly find the include path:
  * PKG_CONFIG_SYSROOT_DIR
  * PKG_CONFIG_LIBDIR

A demo command to generate ninja files are listed below, the content in
angle brackets need to be updated with developer's local env.

PKG_CONFIG_SYSROOT_DIR=<embedded_linux_sdk_sysroot> \
PKG_CONFIG_LIBDIR=${PKG_CONFIG_SYSROOT_DIR}/usr/lib/pkgconfig:${PKG_CONFIG_SYSROOT_DIR}/usr/share/pkgconfig \
gn gen out out/aarch64 --args='target_os="linux" target_cpu="arm64" arm_arch="armv8-a"
      target_cflags=[ "--sysroot=<embedded_linux_sdk_sysroot>" ]
      target_ldflags=[ "--sysroot=<embedded_linux_sdk_sysroot>" ]
      linux_arm_cross_compile="<path_to_cross_compile_tools>"'

Change-Id: Id4b7b4932e16fcc42d77af7020ebb6a7133e237b
Signed-off-by: faqiang.zhu <faqiang.zhu@nxp.com>

* Restyled by gn

* rename the linux_arm toolchain as custom toolchain

The original linux_arm toolchain is not arm specific, it's just a
customizable toolchain. rename it as custom. Do not use this toolchain
based on the target_cpu and target_os, use build argument "custom_toolchain"
to specify this toolchain.

For example, to build with tools in NXP i.MX 8M yocto release SDK, the
command to generate ninja files can be:

PKG_CONFIG_SYSROOT_DIR=<sdk_root_dir> \
PKG_CONFIG_LIBDIR=${PKG_CONFIG_SYSROOT_DIR}/usr/lib/pkgconfig:${PKG_CONFIG_SYSROOT_DIR}/usr/share/pkgconfig \
gn gen out/aarch64 --args='target_os="linux" target_cpu="arm64" arm_arch="armv8-a"
    import("//build_overrides/build.gni")
    target_cflags=[ "--sysroot=<sdk_root_dir>" ]
    target_ldflags=[ "--sysroot=<sdk_root_dir>" ]
    custom_toolchain="${build_root}/toolchain/custom"
    target_cc="<gcc_in_sdk>"
    target_cxx="<g++_in_sdk>"
    target_ar="<ar_in_sdk>"'

Signed-off-by: faqiang.zhu <faqiang.zhu@nxp.com>

Co-authored-by: Restyled.io <commits@restyled.io>

build/toolchain/custom/BUILD.gn

@@ -0,0 +1,45 @@
+# Copyright (c) 2020 Project CHIP Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import("//build_overrides/build.gni")
+import("${build_root}/config/compiler/compiler.gni")
+import("${build_root}/toolchain/gcc_toolchain.gni")
+
+declare_args() {
+  # C compiler to use for target build.
+  target_cc = ""
+
+  # C++ compiler to use for target build.
+  target_cxx = ""
+
+  # Archive tool to use for target build.
+  target_ar = ""
+}
+
+gcc_toolchain("custom") {
+  if (target_cc == "" || target_cxx == "" || target_ar == "") {
+    assert(false,
+           "build arguments target_cc, target_cxx and target_ar need to be set")
+  }
+
+  ar = target_ar
+  cc = target_cc
+  cxx = target_cxx
+
+  toolchain_args = {
+    current_os = target_os
+    current_cpu = target_cpu
+    is_clang = is_clang
+  }
+}