Integration with Buildroot Toolchain Eclipse plugin

The Eclipse plugin at
https://github.com/mbats/eclipse-buildroot-toolchain-plugin

 allows
users of Eclipse to easily use the toolchain available in
Buildroot. To do so, this plugin reads
~/.buildroot-eclipse.toolchains, which contains the list of Buildroot
toolchains available on the system, and then offer those toolchains to
compile Eclipse projects.

In order to interface with this plugin, this commit adds an option
that allows the user to tell whether (s)he wants the Buildroot project
toolchain to be visible under this Eclipse plugin. It simply adds a
line in this ~/.buildroot-eclipse.toolchains file.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>