[Accton] modules: extract accton_ipmi_intf into common kernel-module …

[eric271110]

…source tree

Platforms: as7535-28xb, as7926-40xfb, as7946-30xb, as7946-74xkb, as9737-32db, as9817-64, as9926-24db.

• Move the accton_ipmi_intf.c / accton_ipmi_intf.h kernel-module source that was duplicated in every platform's src/modules tree into a shared source directory under packages/platforms/accton/x86-64/common/modules/src, so all Accton x86-64 platforms build from one copy instead of per-platform duplicates.
• Add a shared build helper kmodule-stage.mk under packages/platforms/accton/x86-64/common/modules. It stages the platform's own src/modules contents plus a caller-supplied list of shared basenames (ACCTON_COMMON_KMOD_SRCS) into a temporary .stage directory and hands that merged tree to kmodbuild.sh via the standard KMODULES variable. Add a README.md next to the helper describing the layout and how a platform opts in.
• Update each platform's modules/builds/Makefile to include kmodule-stage.mk instead of make/kmodule.mk, set ACCTON_COMMON_KMOD_SRCS := accton_ipmi_intf, and change KMODULES to $(CURDIR)/src so the stager can locate the platform-local sources from any working directory. Add .stage to each platform's modules/builds/.gitignore so the staging directory is not tracked.
• Remove the per-platform copies of accton_ipmi_intf.c / accton_ipmi_intf.h under each platform's modules/builds/src (and as9817-64/src/modules) now that the shared source is used.