build: Include config-host.mak as soon as possible

Current code depends on variables defined in config-host.mak before it is
actually included.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Cc: Anthony Liguori <aliguori@us.ibm.com>
Cc: Paul Brook <paul@codesourcery.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>