Let the build script use a default for toolchain

The `cmake` directory already contains several toolchain files for
various platforms (operating system + architecture). However,
`tools/build.py` does not define a toolchain file for cmake unless
explicitly specified. This patch changes the script to look into
the `cmake` directory for a file named
`toolchain_$(os)_$(arch).cmake` and, if found, pass that to cmake
by default.

OS and arch are determined by `os.uname()`. As Linux on Raspberry
Pi identifies itself as "armv7l", the legacy "armv7l-hf" arch name
is shortened to "armv7l". This way, building jerry on RPi
(natively, not cross) becomes possible by simply running
`tools/build.py` without any extra options.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

cmake/toolchain_linux_armv7l.cmake

@@ -0,0 +1,24 @@
+# Copyright 2015-2016 Samsung Electronics Co., Ltd.
+#
+# 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 Linux)
+set(CMAKE_SYSTEM_PROCESSOR armv7l)
+
+set(CMAKE_C_COMPILER arm-linux-gnueabihf-gcc)
+#
+# Limit fpu to VFPv3 with d0-d15 registers
+#
+# If this is changed, setjmp / longjmp for ARMv7 should be updated accordingly
+#
+set(FLAGS_COMMON_ARCH -mthumb -mfpu=vfp -march=armv7)