cmake: compute and display the reproducible checksum by default

Temporary bugs, corner cases and obsolete toolchains aside, the Zephyr
build is reproducible most of the time: zephyrproject-rtos#50205 and zephyrproject-rtos#14593

This means two different build machines using the same toolchain will
always produce the same binary output. The previous, one-line commit made
it trivial to verify that binary outputs are indeed the same by adding this
single line in the buid logs:

```
[16/16] Linking C executable zephyr/zephyr.elf
Memory region         Used Size  Region Size  %age Used
             RAM:       53280 B         3 MB      1.69%
        IDT_LIST:          0 GB         2 KB      0.00%
fdd2ddf2ad7d5da5bbd79b41cef8d7...1a896b989a8281111d8e205  zephyr.strip
```

This commit enables that feature by default because build
reproducibility matters for (at least) two important reasons:

- Security / supply chain attacks, see https://www.cisa.gov/sbom, zephyrproject-rtos#50205,
  https://reproducible-builds.org/ and many others.
- Making sure build configurations are strictly identical when trying to
  reproduce elusive issues or when issuing releases.

It was of course already possible to _manually_ make this Kconfig change
and manually compute this checksum. However this can be impossible when
dealing with an automated build system that does not archive all
_intermediate_ (zephyrproject-rtos#5009) files like `zephyr.elf`. Tweaking the build
configuration can also be difficult and error-prone for people who are
not Zephyr developers.

Most automated CI systems preserve build logs by default.

Displaying the reproducible checksum by default accelerates the
discovery of reproducibility bugs like zephyrproject-rtos#48195.

When measured with `west build -p -b qemu_x86 samples/hello_world/`, the
additional `build/zephyr/zephyr.strip` disk space required is 43
kilobytes compared to a total of 11 Megabytes. Measuring a more
realistic SOF example, `zephyr.strip` weighed 690 kb which was about
0.1% of a total `build/` directory weighing 65M.

To measure the build time cost I ran `west build -p -b qemu_x86
samples/hello_world/` many times in a loop with and without this PR on
my Linux workstation. Stripping and checksumming made literally no time
difference compared to the "noise" observed when building the same
configuration. This is not surprising considering how small
`zephyr.strip`: so the extra cost is most likely dominated by process
creation and the total number of processes created during a Zephyr build
dwarfs the few extra processes required by this feature.

More surprisingly, I measured incremental builds by running `touch
kernel/timer.c; west build ...` in a loop and I could not observe any
visible time difference either.

Signed-off-by: Marc Herbert <[email protected]>

Kconfig.zephyr

@@ -575,6 +575,7 @@ endif # BUILD_OUTPUT_UF2
 
 config BUILD_OUTPUT_STRIPPED
 	bool "Build a stripped binary"
+	default y
 	help
 	  Build a stripped binary zephyr/zephyr.strip in the build directory.
 	  The name of this file can be customized with CONFIG_KERNEL_BIN_NAME.