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There was an OpenWrt toolchain file however it's a bit outdated and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)