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/nuttx-stm32f4/README.md

@@ -42,7 +42,7 @@ jerryscript/tools/build.py \
     --lto=OFF \
     --jerry-cmdline=OFF \
     --jerry-math=ON \
-    --all-in-one=ON \
+    --amalgam=ON \
     --mem-heap=70 \
     --profile=es.next \
     --compile-flag="--sysroot=${PWD}/nuttx" \