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

targets/nuttx-stm32f4/Makefile.nuttx

@@ -40,7 +40,7 @@ all:
 	 -DENABLE_LTO=OFF \
 	 -DENABLE_VALGRIND=OFF \
 	 -DCMAKE_TOOLCHAIN_FILE=build/configs/toolchain_external.cmake \
-	 -DEXTERNAL_CMAKE_SYSTEM_PROCESSOR=armv7l-hf \
+	 -DEXTERNAL_CMAKE_SYSTEM_PROCESSOR=armv7l \
 	 -DEXTERNAL_CMAKE_C_COMPILER=arm-none-eabi-gcc \
 	 -DEXTERNAL_CMAKE_C_COMPILER_ID=GNU \
 	 -DEXTERNAL_BUILD_ENTRY_FILE=./targets/nuttx-stm32f4/main-nuttx.c \

targets/riot-stm32f4/Makefile.riot

@@ -37,7 +37,7 @@ libjerry:
 	 -DJERRY_CMDLINE=OFF \
 	 -DCOMPILER_DEFAULT_LIBC=ON \
 	 -DENABLE_ALL_IN_ONE=OFF \
-	 -DEXTERNAL_CMAKE_SYSTEM_PROCESSOR=armv7l-hf \
+	 -DEXTERNAL_CMAKE_SYSTEM_PROCESSOR=armv7l \
 	 -DEXTERNAL_CMAKE_C_COMPILER=arm-none-eabi-gcc \
 	 -DEXTERNAL_CMAKE_C_COMPILER_ID=GNU \
 	 -DEXTERNAL_COMPILE_FLAGS="$(EXT_CFLAGS)" \