[build] Use $(eval) if available

[build] Use $(eval) if available

When the $(eval) function is available (in GNU make >= 3.80), we can
evaluate many of the dynamically-generated Makefile rules directly.
This avoids generating a few hundred Makefile fragments in the
filesystem, and so speeds up the build process.

Signed-off-by: Michael Brown <mcb30@ipxe.org>