bcp --namespace breaks path to import boost_install in Jamroot

When using bcp.exe to create a boost subset with a new namespace, building the subset is broken because in the Jamfile the import for boost_install has had the "boost_" portion renamed to the new namespace.

```
Build\boost>.\dist\bin\bcp.exe --namespace=myboost --namespace-alias regex log headers predef config build boost_install ..\foo
Build\boost>cd ..\foo
Build\foo>.\bootstrap.bat
Build\foo>.\b2 toolset=msvc-14.1 --with-regex --with-log
boost-install.jam: No such file or directory
Jamroot:307: in boost-install
ERROR: rule "boost-install.boost-install" unknown in module "Jamfile".
libs\atomic\build\Jamfile.v2:38: in modules.load
```