[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>
[romprefix] Fix incorrect pointer offset in undiloader.S

Commit 2422647 ("[prefix] Allow prefix to specify an arbitrary maximum
address for relocation") introduced a regression into the UNDI ROM
loader by preserving an extra register on the stack without modifying
the %sp-relative addresses used in the routine.

Fix by correcting the %sp-relative addresses to allow for the extra
preserved variable.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>