build/boot/roms: fix coreboot-version in releases

This error was observed, in the coreboot build system:

In file included from src/lib/version.c:4:
build/build.h:10:32: error: 'libreboot' undeclared here (not in a function)
   10 | #define COREBOOT_MAJOR_VERSION libreboot-20230625
      |                                ^~~~~~~~~
src/lib/version.c:35:46: note: in expansion of macro 'COREBOOT_MAJOR_VERSION'
   35 | const unsigned int coreboot_major_revision = COREBOOT_MAJOR_VERSION;
      |                                              ^~~~~~~~~~~~~~~~~~~~~~

This happened on the 20230625 *release archive*, when a user tried to
build for W541 MRC on an Arch Linux container.

This change fixes the error. I never got the error on my end when
build testing the release archives, but this will prevent the error.
Fix it by only inserting libreboot version string YYYYMMDD representing
the Libreboot version. (libreboot uses ISO dates as version numbers)

Signed-off-by: Leah Rowe <leah@libreboot.org>