docs/manual: rephrase paragraph about unsupported external toolchains

The ELDK now relies on Yocto, so it no longer has the funky
non-standard way of doing multilib. Also, we didn't mention that Yocto
toolchains/SDK are not suitable for usage with Buildroot. Therefore,
this commit rewords this part of the documentation to explain that
Yocto/OpenEmbedded SDKs cannot be used as external toolchains in
Buildroot, and why.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>