Deprecate unused scripts in Tools/buildbot.

I would just outright delete them, but the readme in PCbuild recommended
their use, so I figure it would be nice to leave them there for a while.

Tools/buildbot/build-amd64.bat

@@ -1,2 +1,5 @@
-@rem Used by the buildbot "compile" step.
+@rem Formerly used by the buildbot "compile" step.
+@echo This script is no longer used and may be removed in the future.
+@echo To get the same effect as this script, use
+@echo     PCbuild\build.bat -d -e -k -p x64
 call "%~dp0build.bat" -p x64 %*

Tools/buildbot/clean-amd64.bat

@@ -1,2 +1,5 @@
-@rem Used by the buildbot "clean" step.
+@rem Formerly used by the buildbot "clean" step.
+@echo This script is no longer used and may be removed in the future.
+@echo To get the same effect as this script, use `clean.bat` from this
+@echo directory and pass `-p x64` as two arguments.
 call "%~dp0clean.bat" -p x64 %*

Tools/buildbot/external-amd64.bat

@@ -1,2 +1,3 @@
+@echo This script is no longer used and may be removed in the future.
 @echo Please use PCbuild\get_externals.bat instead.
 @"%~dp0..\..\PCbuild\get_externals.bat" %*

Tools/buildbot/external.bat

@@ -1,2 +1,3 @@
+@echo This script is no longer used and may be removed in the future.
 @echo Please use PCbuild\get_externals.bat instead.
 @"%~dp0..\..\PCbuild\get_externals.bat" %*

Tools/buildbot/test-amd64.bat

@@ -1,2 +1,6 @@
-@rem Used by the buildbot "test" step.
+@rem Formerly used by the buildbot "test" step.
+@echo This script is no longer used and may be removed in the future.
+@echo To get the same effect as this script, use
+@echo     PCbuild\rt.bat -q -d -x64 -uall -rwW
+@echo or use `test.bat` in this directory and pass `-x64` as an argument.
 call "%~dp0test.bat" -x64 %*