Add basic config infrastructure for host utilities

Most of the host packages don't have to be exposed to the user as they
are only used as build dependencies of target packages.

However, some host utilities, such as flashing utilities, image
creation programs, specific debuggers, might be useful and should be
presented to the user.

Therefore, we have a new global menu, which lists those host
utilities. These utilities are described in package/*/Config.in.host
files, which will be sourced by package/Config.in.host.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>