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/zephyr/Makefile.zephyr

@@ -77,21 +77,19 @@ endif
 	mkdir -p $(INTERM)
 	mkdir -p $(OUTPUT)
 	cmake -B$(INTERM) -H./ \
- 	 -DENABLE_LTO=OFF \
-	 -DFEATURE_VALGRIND=OFF \
+	 -DCMAKE_SYSTEM_NAME=Zephyr \
+	 -DCMAKE_C_COMPILER="$(subst ccache ,,$(CC))" \
+	 -DCMAKE_C_COMPILER_WORKS=TRUE \
+	 -DENABLE_LTO=OFF \
+	 -DENABLE_ALL_IN_ONE=OFF \
+	 -DJERRY_LIBC=OFF \
+	 -DJERRY_CMDLINE=OFF \
 	 -DFEATURE_PROFILE=$(JERRYPROFILE) \
 	 -DFEATURE_ERROR_MESSAGES=ON \
 	 -DCMAKE_BUILD_TYPE=MinSizeRel \
 	 -DCMAKE_VERBOSE_MAKEFILE=$(V) \
 	 -DMEM_HEAP_SIZE_KB=$(JERRYHEAP) \
-	 -DEXTERNAL_CMAKE_C_COMPILER="$(subst ccache ,,$(CC))" \
-	 -DEXTERNAL_CMAKE_C_COMPILER_ID=GNU \
-	 -DJERRY_CMDLINE=OFF \
 	 -DEXTERNAL_COMPILE_FLAGS="$(EXT_CFLAGS)" \
-	 -DCMAKE_TOOLCHAIN_FILE=cmake/toolchain_external.cmake \
-	 -DFEATURE_SNAPSHOT_EXEC=OFF \
-	 -DENABLE_ALL_IN_ONE=OFF \
-	 -DJERRY_LIBC=OFF \
 	 $(EXT_JERRY_FLAGS)
 
 	make -C $(INTERM) $(COMPONENTS) V=1