Documentation: kbuild: note CONFIG_DEBUG_EFI in reproducible builds

CONFIG_EFI_DEBUG embeds absolute file paths into object files, which
makes the resulting vmlinux specific to the build environment.

Add a note about this in reproducible-builds.rst

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Nicolas Schier <nsc@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20250924-kbuild-doc-config-debug-efi-in-reproducible-builds-v2-1-d2d6b9dcdb7d@kernel.org


Signed-off-by: Nathan Chancellor <nathan@kernel.org>