build: build host-tools when prebuilts are enabled

The build process of the host tools requires fixdep, when prebuilt host
tools are enabled and no host-tools are available (i.e. after a 'make
mrproper'), the build would fail because of the dependency. So make sure
we point to the in tree fixdep binary when building.

Change-Id: I8311f870d90b32ba56c821bb8533379f57003a8d
Signed-off-by: Anas Nashif <anas.nashif@intel.com>