Add first sample for MIPS cross compile

cmake/Toolchain-Linux-openwrt-mips.cmake.sample

+# Toolchain file for building with OpenWRT Toolchain for MIPS.
+# This is a preliminary version for mips outside of the buildroot-ng.
+# The next version should be target independent.
+# Further improvements:
+#   - use ENV variables in OpenWRT toolchain (like HOSTCC, TARGET_CC)
+#   - set flags properly for host and target
+
+set(CMAKE_SYSTEM_NAME Linux)
+set(TOOLCHAIN_PREFIX mips-openwrt-linux)
+set(CMAKE_C_COMPILER ${TOOLCHAIN_PREFIX}-gcc)