build directory outside of Genode dir xor vim error

[Paul Dufresne]

And make run/noux_bash on okl4:
    COMPILE  main.o
In file included from /home/paul/mygenode/base/src/core/main.cc:24:0:
/home/paul/mygenode/base/src/core/include/core_env.h:121:33: error:
cannot declare field ‘Genode::Core_env::_rm_session’ to be of abstract
type ‘Genode::Core_rm_session’
In file included from
/home/paul/mygenode/base-okl4/src/core/include/platform.h:24:0,
                 from /home/paul/mygenode/base/src/core/main.cc:23:
/home/paul/mygenode/base-okl4/src/core/include/core_rm_session.h:30:8:
note:   because the following virtual functions are pure within
‘Genode::Core_rm_session’:
In file included from /home/paul/mygenode/base/include/base/env.h:22:0,
                 from /home/paul/mygenode/base/include/base/service.h:21,
                 from /home/paul/mygenode/base/src/core/main.cc:17:
/home/paul/mygenode/base/include/rm_session/rm_session.h:172:16: note:
	virtual void Genode::Rm_session::remove_client(Genode::Pager_capability)
make[3]: *** [main.o] Error 1
make[2]: *** [core.prg] Error 2
make[1]: *** [gen_deps_and_build_targets] Error 2
make[1]: Leaving directory `/home/paul/build.okl4'
Error: Genode build failed
make: *** [run/noux_bash] Error 252
paul at ...162...:~/build.okl4$

While trying the different 'platforms' (kernels), the build system
seems a bit unoptimized.
To me, rebuilding libports, ports, gems in each different kernels
don't make much sense.
I don't expect them to apply patch platform-specifics, only Genode-specifics.
So it would make sense to specify two directory on create_builddir:
one common for all platforms, and one for the platform.