toolchain: introduce HOST_DIR option

Allow the user to define HOST_DIR in the config menu.

This way when building an internal toolchain a separate (maybe shared)
output directory may be defined and the toolchain can be used by
multiple users and/or projects.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>