build: support pre-built host tools

Right now the build system builds the host tools over and over again, in some
environments especially when running in an IDE on windows for example,
this is not desired and a set of pre-built host tools should be used.

Provide an option to use pre-built tools instead of building them
from source.

To use, set PREBUILT_HOST_TOOLS to the path where all pre-built host tools
are hosted. To get a prebuilt version of the host tools, build without the
variable set and copy the generated host binaries from outdir. The following
tools are supported:

* conf
* fixdep
* gen_idt
* gen_offset_header

Jira: ZEP-237
Change-Id: Iea505bfd0b50f851ee2781b5117bb6085ab20157
Signed-off-by: Anas Nashif <anas.nashif@intel.com>