Kconfig,Makefile.inc: Add support for link time optimization

Link time optimization is a technique for whole-program optimization.
Instead of doing code generation during compilation, the compiler saves
its intermediate representation to the object files. During the final
linking step, it will then merge all the object files together and
perform optimizations on the entire program. This can often reduce the
final binary size, but also may increase the total compilation time.

This patch introduces a Kconfig option for enabling link time
optimization in coreboot. Due to the potential for breakage, this option
is disabled by default, and would only be enabled per-board once it has
been tested on hardware. Not all targets compile now, but early results
are encouraging: using LTO leads to a ~13% decrease in stage size for
QEMU i440fx and ~10% for the Thinkpad T500, and both targets boot successfully.
Compile times for both are also about 10% faster.

Change-Id: I48c31ea8b1b57276125cffdac44c7c16642547ac
Signed-off-by: Jacob Garber <jgarber1@ualberta.ca>
---
M Makefile.inc
M src/Kconfig
2 files changed, 12 insertions(+), 0 deletions(-)