util/scripts: Add script to show platforms, CPU, type, and date added

This is the script used to generate the list of platforms that were
removed from the master branch at each release.  Generate a list for the
old branch, another for the new, and compare the two.

Representative output:
```eval_rst
+-------------------------+-------------------+------------+----------+
| Vendor/Board            | Processor         | Date added | Brd type |
+=========================+===================+============+==========+
| 51nb/x210               | INTEL_KABYLAKE    | 2020-03-16 | laptop   |
| acer/aspire_vn7_572g    | INTEL_SKYLAKE     | 2022-01-28 | laptop   |
| acer/g43t-am3           | INTEL_X4X         | 2020-09-28 | desktop  |
| amd/bilby               | AMD_PICASSO       | 2021-02-17 | eval     |
| amd/birman              | AMD_MORGANA       | 2022-10-10 | eval     |
| system76/whl-u          | INTEL_WHISKEYLAKE | 2021-04-14 | laptop   |
| ti/beaglebone           | TI_AM335X         | 2013-05-26 | sbc      |
| up/squared              | INTEL_APOLLOLAKE  | 2019-05-22 | mini     |
+-------------------------+-------------------+------------+----------+
```

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: I4f7265d95df31f3a74aa2aa164f6a094c1139750
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63799
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

util/scripts/description.md

@@ -25,5 +25,8 @@ __scripts__
                              `Perl`
   * _rm_unused_code_ - Remove all code not used for a platform from the local
                  git repository for auditing or release `Bash`
+  * _show_platforms.sh_ - Makes a list of platforms in the tree. Does
+                          not show variants.
+                          `Shell`
   * _ucode_h_to_bin.sh_ - Microcode conversion tool `Bash`
   * _update_submodules_ - Check all submodules for updates `Bash`