Remove the 'external' toolchain file (#2089)

The 'external' toolchain file does nothing but transitively sets
some cmake system variables from values received on the command
line, and forcibly sets the C compiler. However, the same cmake
system variables can be directly set via the command line, together
with the C compiler, and specifying a toolchain is not a must.
Thus, this patch drops the superfluous 'external' toolchain file
and updates cmake-based targets to invoke cmake in a simpler form.

Related changes in this commit:
- While updating the cmake invocations, all the command line
  arguments have been reviewed and simplified (removed those, which
  did not change the defaults).
- Removed unnecessary forced C compiler settings from some
  toolchain files (and/or changed them to setting the "compiler
  works" flag to true, thus keeping cmake's compiler identification
  logic but disabling some of its overzealous compiler sanity
  checks).

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

targets/mbed/Makefile.mbed

@@ -53,15 +53,14 @@ jerry:
 	mkdir -p $(COPYTARGET)
 	mkdir -p $(BUILD_DIR)
 	cmake -B$(BUILD_DIR) -H./ \
+	 -DCMAKE_SYSTEM_NAME=Mbed \
+	 -DCMAKE_SYSTEM_PROCESSOR=armv7l-hf \
+	 -DCMAKE_C_COMPILER=arm-none-eabi-gcc \
+	 -DCMAKE_C_COMPILER_WORKS=TRUE \
 	 -DENABLE_LTO=OFF \
-	 -DFEATURE_VALGRIND=OFF \
-	 -DCMAKE_TOOLCHAIN_FILE=cmake/toolchain_external.cmake \
+	 -DENABLE_ALL_IN_ONE=OFF \
 	 -DJERRY_LIBC=OFF \
 	 -DJERRY_CMDLINE=OFF \
-	 -DENABLE_ALL_IN_ONE=OFF \
-	 -DEXTERNAL_CMAKE_SYSTEM_PROCESSOR=armv7l-hf \
-	 -DEXTERNAL_CMAKE_C_COMPILER=arm-none-eabi-gcc \
-	 -DEXTERNAL_CMAKE_C_COMPILER_ID=GNU \
 	 -DEXTERNAL_COMPILE_FLAGS="$(EXT_CFLAGS)" \
 	 -DMEM_HEAP_SIZE_KB=$(JERRYHEAP) \