Add OpenWrt build guide (#2199)

There was an OpenWrt toolchain file however it's a bit outdated and
we did not have any guide on how to build for this target at all.

JerryScript-DCO-1.0-Signed-off-by: Peter Gal pgal.u-szeged@partner.samsung.com

cmake/toolchain_openwrt_mips.cmake

@@ -15,4 +15,4 @@
 set(CMAKE_SYSTEM_NAME Openwrt)
 set(CMAKE_SYSTEM_PROCESSOR mips)
 
-set(CMAKE_C_COMPILER mipsel-openwrt-linux-gcc)
+set(CMAKE_C_COMPILER mips-openwrt-linux-gcc)

cmake/toolchain_openwrt_mipsel.cmake

@@ -0,0 +1,18 @@
+# Copyright JS Foundation and other contributors, http://js.foundation
+#
+# 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.
+
+set(CMAKE_SYSTEM_NAME Openwrt)
+set(CMAKE_SYSTEM_PROCESSOR mips)
+
+set(CMAKE_C_COMPILER mipsel-openwrt-linux-gcc)

targets/openwrt/readme.md

@@ -0,0 +1,100 @@
+# JerryScript for OpenWrt build guide
+
+This document describes the steps required to compile the JerryScript
+for OpenWrt. For target device the TP-Link WR1043ND v1.x router is
+used. Please be advised, that if you have a different one minor
+modifications to this document could be required.
+
+IMPORTANT!
+
+As the TP-Link WR1043ND is a mips based device and mips is a big-endian
+architecture a JerryScipt snapshot which was built on an little-endian
+system will not work correctly. Thus it is advised that the
+snapshot functionally should be used with caution, that is
+DO NOT run snapshots generated on little-endian system(s) on
+a big-endian system.
+
+## OpenWrt notes
+
+In 2018 ~January the OpenWrt and LEDE project merged into one
+and thus the old OpenWrt parts are now usable only from
+an archived repository: https://github.com/openwrt/archive
+
+## OpenWrt toolchain setup
+
+To build the JerryScript for OpenWrt a toolchain is required for
+the target router/device. The toolchain setup in this document was
+tested on an Ubuntu 16.04.3 LTS Linux.
+
+Steps required for toolchain creation:
+
+### 0. Install OpenWrt build requirements
+```sh
+$ sudo apt-get install git-core build-essential libssl-dev libncurses5-dev unzip gawk zlib1g-dev subversion mercurial
+```
+
+### 1. Clone OpenWrt (Chaos Calmer version)
+
+```sh
+$ git clone https://github.com/openwrt/archive openwrt -b chaos_calmer
+$ cd openwrt
+```
+
+### 2. Run Menuconfig and configure the OpenWrt
+
+```sh
+$ make menuconfig
+```
+
+Options which should be set:
+* Set "Target System" to "Atheros AR7xxx/AR9xxx".
+* Set "Target Profile" to "TP-LINK TL-WR1043N/ND".
+
+Save the configuration (as .config) and exit from the menuconfig.
+
+### 3. Configure the environment variables
+
+```sh
+$ export BUILDROOT=$(pwd) # where the openwrt root dir is
+$ export STAGING_DIR=${BUILDROOT}/staging_dir/ # required by the compiler
+$ export PATH=$PATH:${STAGING_DIR}/host/bin:${STAGING_DIR}/toolchain-mips_34kc_gcc-4.8-linaro_uClibc-0.9.33.2/bin/
+```
+
+The name `toolchain-mips_34kc_gcc-4.8-linaro_uClibc-0.9.33.2` is created based on the menuconfig.
+This changes depending on the target device!
+
+### 4. Build the OpenWrt
+
+```sh
+$ make
+```
+
+### 5. Check if the compiler was built
+
+```sh
+$ mips-openwrt-linux-gcc --version # running this should print out the version information
+```
+
+At this point we have the required compiler for OpenWrt.
+
+## Build JerryScript for OpenWrt
+
+### 0. Check environment
+
+Please check if the `STAGING_DIR` is configured correctly and that the toolchain binary is on the `PATH`.
+
+### 1. Run the build with the OpenWrt toolchain file
+
+```
+$ ./tools/build.py --toolchain cmake/toolchain_openwrt_mips.cmake \
+                   --jerry-libc OFF \
+                   --lto OFF
+```
+
+Currenlty the JerryScript libc does not supports the mips platform, that's why the `--jerry-libc OFF` argument
+is passed during build.
+
+### 2. Copy the binary
+
+After a successful build the `build/bin/jerry` binary file can be copied to the target device.
+On how to copy a binary file to an OpenWrt target device please see the OpenWrt manual(s).