Officially introduce amalgamated builds (#4416)

Remove redundancy between all-in-one and all-in-one-source builds
by keeping only the second, and adopt the more established term
"amalgamated" build for it. This change includes the following:

- Replace `ENABLE_ALL_IN_ONE` and `ENABLE_ALL_IN_ONE_SOURCE` cmake
  options with `ENABLE_AMALGAM` top-level option.
- Replace `--all-in-one` option of `build.py` helper with
  `--amalgam`.
- Merge the `srcmerger.py` and `srcgenerator.py` tool scripts into
  `amalgam.py` (with improvements).
- Update documentation.

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

targets/zephyr/Makefile.zephyr

@@ -82,7 +82,6 @@ endif
 	 -DCMAKE_C_COMPILER="$(subst ccache ,,$(CC))" \
 	 -DCMAKE_C_COMPILER_WORKS=TRUE \
 	 -DENABLE_LTO=OFF \
-	 -DENABLE_ALL_IN_ONE=OFF \
 	 -DJERRY_CMDLINE=OFF \
 	 -DJERRY_PROFILE=$(JERRYPROFILE) \
 	 -DJERRY_ERROR_MESSAGES=ON \