scripts: create a separate section for host tools ...

This allows to enable host tools even if they are not needed for the
current configuration to improve compile coverage and simplify packaging
these tools. The conversion doesn't cover all tools available but can be
extended later.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>