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

tools/run-tests.py

@@ -129,8 +129,8 @@ JERRY_BUILDOPTIONS = [
             ['--error-messages=on']),
     Options('buildoption_test-logging',
             ['--logging=on']),
-    Options('buildoption_test-all_in_one',
-            ['--all-in-one=on']),
+    Options('buildoption_test-amalgam',
+            ['--amalgam=on']),
     Options('buildoption_test-valgrind',
             ['--valgrind=on']),
     Options('buildoption_test-mem_stats',
@@ -163,8 +163,6 @@ JERRY_BUILDOPTIONS = [
             OPTIONS_STACK_LIMIT),
     Options('buildoption_test-gc-mark_limit',
             OPTIONS_GC_MARK_LIMIT),
-    Options('buildoption_test-single-source',
-            ['--cmake-param=-DENABLE_ALL_IN_ONE_SOURCE=ON']),
     Options('buildoption_test-jerry-debugger',
             ['--jerry-debugger=on']),
     Options('buildoption_test-module-off',