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

.travis.yml

@@ -16,7 +16,7 @@ script: "python tools/run-tests.py $OPTS"
 env:
   - OPTS="--check-signed-off --check-cppcheck --check-vera"
   - OPTS="--jerry-tests --jerry-test-suite"
-  - OPTS="--jerry-tests --jerry-test-suite --toolchain=cmake/toolchain_linux_armv7l-hf.cmake" TIMEOUT=300
+  - OPTS="--jerry-tests --jerry-test-suite --toolchain=cmake/toolchain_linux_armv7l.cmake" TIMEOUT=300
   - OPTS=--buildoption-test
   - OPTS=--unittests