[M] Change in coreboot[master]: libpayload: Enable LTO by default

[Paul Menzel (Code Review)]

Attention is currently required from: Nico Huber, Stefan Reinauer, Paul Menzel, Julius Werner.

Hello build bot (Jenkins), Nico Huber, Angel Pons, Julius Werner,

I'd like you to reexamine a change. Please visit

https://review.coreboot.org/c/coreboot/+/51177

to look at the new patch set (#13).

Change subject: libpayload: Enable LTO by default
......................................................................

libpayload: Enable LTO by default

Link Time Optimization (LTO) decreases the size of the binaries, which
is important for firmware with small flash ROM chip sizes. Using LTO
might result in longer build times, which is not an issue for a small
project like libpayload.

Therefore, enable LTO by default.

TEST=Build coreinfo without and with LTO and test with
emulation/qemu-i440fx.

qemu-system-x86_64 -bios coreboot.rom -L /dev/shm -enable-kvm -smp cpus=2 -m 1G -hda /dev/shm/debian-32.img -serial stdio -net nic -net user,hostfwd=tcp::22222-:22

```
$ make -j4 # no LTO
[…]
    LPGCC      coreinfo.bin
/usr/bin/ld.bfd: warning: exception_asm.libc.o: missing .note.GNU-stack section implies executable stack
/usr/bin/ld.bfd: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
/usr/bin/ld.bfd: warning: build/coreinfo.bin has a LOAD segment with RWX permissions
    DEBUG      coreinfo.debug
    STRIP      coreinfo.elf

real	0m9,101s
user	0m8,754s
sys	0m1,526s
$ ls -ltra build/coreinfo.{bin,debug,elf}
-rwxr-xr-x 1 user user 648248 10. Nov 19:00 build/coreinfo.bin
-rwxr-xr-x 1 user user 545704 10. Nov 19:00 build/coreinfo.debug
-rwxr-xr-x 1 user user 130788 10. Nov 19:00 build/coreinfo.elf
```

```
$ make -j4 # with LTO
[…]
    LPGCC      coreinfo.bin
lto-wrapper: warning: using serial compilation of 2 LTRANS jobs
lto-wrapper: note: see the '-flto' option documentation for more information
/usr/bin/ld.bfd: warning: exception_asm.libc.o: missing .note.GNU-stack section implies executable stack
/usr/bin/ld.bfd: NOTE: This behaviour is deprecated and will be removed in a future version of the linker
/usr/bin/ld.bfd: warning: build/coreinfo.bin has a LOAD segment with RWX permissions
    DEBUG      coreinfo.debug
    STRIP      coreinfo.elf

real	0m6,675s
user	0m6,399s
sys	0m1,395s
$ ls -ltra build/coreinfo.{bin,debug,elf}
-rwxr-xr-x 1 user user 616612 10. Nov 19:01 build/coreinfo.bin
-rwxr-xr-x 1 user user 518340 10. Nov 19:01 build/coreinfo.debug
-rwxr-xr-x 1 user user 125988 10. Nov 19:01 build/coreinfo.elf
```

So real time usage actually decreased by 33 % from nine to six seconds,
and size of the ELF by 4 % from 130.8 to 126.0 bytes

Change-Id: I4bc58034ea8a9510ddc08d61d19e75a7c6f9cbc7
Signed-off-by: Paul Menzel 
---
M payloads/libpayload/Kconfig
1 file changed, 66 insertions(+), 1 deletion(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/77/51177/13