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
