feat: restructure repo with per-kernel-version branches (v5.4 through v6.12)

[bacnh85]

Summary

Restructured the repository with dedicated branches for each LTS kernel version. Users checkout the branch matching their kernel, then choose between out-of-tree module build or in-kernel patching.

New Branch Structure

Branch	Kernel	Quectel Serial Devices	QMI Entries	Backport Size
v5.4	5.4.x LTS	+14 products from 6.12	+12 entries	702-line option.c patch
v5.15	5.15.x LTS	+12 products from 6.12	+8 entries	502-line option.c patch
v6.1	6.1.x LTS	+10 products from 6.12	+4 entries	374-line option.c patch
v6.6	6.6.x LTS	+6 products from 6.12	+2 entries	265-line option.c patch
v6.12	6.12.x	Full mainline support	Full support	No patch needed

What Changed

• Removed old patched drivers and PDF documentation
• Added per-kernel branches with:
  • Out-of-tree driver sources (QuectelWB with 3.x–6.x+ compat wrappers)
  • In-kernel patches (diff from vanilla kernel → 6.12 Quectel support)
  • qmi_wwan_q.c with QMAP multiplexing support
  • Makefile for out-of-tree make build
  • Comprehensive README.md with both build methods

Two Build Methods (per branch)

1. Out-of-tree: make && sudo make install — builds option.ko, usb_wwan.ko, qmi_wwan_q.ko
2. In-kernel patch: patch -p1 < patches/option.c.patch — applies Quectel device ID backport to kernel source

Supported Quectel Modules

UC15, UC20, EC20, EC21, EC25, EG91, EG95, BG96, BG95, AG35, EP06, EM05G series, EM060K/EM061K series, EM12, EM160R-GL, RM500Q, RM520N, RM500K, RM500U, RG650V, EC200S, EC200A, EC200T, EC200U, EG912Y, EG916Q

Source Attribution

• Out-of-tree: QuectelWB/q_drivers (GPL-2.0)
• Patches: Backported from mainline Linux 6.12 to older LTS kernels
• No dependency on Quectel proprietary instructions

[bacnh85]

Update: PR is now in a merge-ready state.

What changed

• Switched to an upstream-first, patch-driven structure
• Kept upstream-style option.c / qmi_wwan.c instead of vendor-specific qmi_wwan_q.c
• Reworked out-of-tree build flow to copy kernel sources from KDIR, apply branch-specific patches, then build
• Fixed patch generation/format issues so GNU patch validates cleanly
• Fixed Makefile path handling, patch strip levels, and header copying (usb-wwan.h)
• Hardened GitHub Actions so kernel build failures are surfaced correctly instead of hidden by piped tail
• Added compatibility handling for older kernels (notably v5.4 QMI patch compatibility)
• For v5.15, pinned CI kernel tooling to gcc-11/g++-11 to avoid the older-kernel objtool failure on newer GCC

Current status

• v5.4: passing
• v5.15: passing
• v6.1: passing
• v6.6: passing
• v6.12: passing

Notes

• PR head (v6.12) is green
• PR merge state is clean
• Branches remain structured per target kernel version as requested
• Approach remains upstream-preferred and suitable for merge to master

If desired after merge, a follow-up cleanup can squash some of the iterative CI-fix commits, but functionally the PR is ready.