doc/boot-process.md: document Layer 1 framebuffer driver check

Layer 1 now also checks each initrd for known framebuffer drivers
(efifb on all boards, plus bochs on QEMU) and writes results to
/tmp/kexec_efifb_compat.txt.  A separate warning is shown if no
initrd has a known fb driver.

Author: tlaurion

doc/boot-process.md

@@ -131,24 +131,28 @@ boot flow. It is invoked from the "USB ISO Boot" option in the main menu.
 
 1. **Signature verification**: Check for `.sig` or `.asc` detached signature
 2. **Mount ISO**: Mount the ISO file as loopback device at `/boot`
-3. **Layer 1 — initramfs fs compatibility check** (`check_initrd_fs_compat`):
-   Before presenting boot options, verify the ISO's initramfs contains kernel
-   modules for the USB partition's filesystem (ext4/vfat/exfat).  If the initrd
-   can't read the USB filesystem, the kernel won't find the ISO after kexec.
-   - Parsing boot configs for initrd paths (instead of searching the whole ISO)
-   - Unpacking each initrd and checking for required `.ko` files and
-     `modules.builtin`
-   - Each initrd gets its own independent `[OK]` / `[!]` / (blank) marker in
-     `/tmp/kexec_initrd_compat.txt` (the per-initrd flag `initrd_supports_fs` is tracked
-     separately from the global `any_supported` flag, so no initrd is silently
-     skipped)
-   - `[OK]` = initrd has the needed module as `.ko`, has it in
-     `modules.builtin`, or has no `.ko` files at all (minimal initrd with
-     everything built into the kernel — nothing to check against).
-   - `[!]`  = initrd has loadable kernel modules but none for the USB
-     filesystem type.  No built-in assumption — we report what we find.
-   - Read-only filesystems (iso9660/squashfs/udf) and unmapped fstypes skip
-   - All initrds are checked (no early break) so the compat file is complete.
+ 3. **Layer 1 — initramfs fs compatibility check** (`check_initrd_fs_compat`):
+    Before presenting boot options, verify the ISO's initramfs contains kernel
+    modules for the USB partition's filesystem (ext4/vfat/exfat).  If the initrd
+    can't read the USB filesystem, the kernel won't find the ISO after kexec.
+    Also checks for a framebuffer driver (efifb, bochs) needed for display after
+    kexec.
+    - Parsing boot configs for initrd paths (instead of searching the whole ISO)
+    - Unpacking each initrd and checking for required `.ko` files and
+      `modules.builtin`
+    - Each initrd gets its own independent `[OK]` / `[!]` / (blank) marker in
+      `/tmp/kexec_initrd_compat.txt` (the per-initrd flag `initrd_supports_fs` is tracked
+      separately from the global `any_supported` flag, so no initrd is silently
+      skipped)
+    - `[OK]` = initrd has the needed module as `.ko`, has it in
+      `modules.builtin`, or has no `.ko` files at all (minimal initrd with
+      everything built into the kernel — nothing to check against).
+    - `[!]`  = initrd has loadable kernel modules but none for the USB
+      filesystem type.  No built-in assumption — we report what we find.
+    - Read-only filesystems (iso9660/squashfs/udf) and unmapped fstypes skip
+    - All initrds are checked (no early break) so the compat file is complete.
+    - Framebuffer results are written to `/tmp/kexec_efifb_compat.txt`.  A
+      separate warning is shown if no initrd has a known fb driver.
 4. **Layer 2 — loopback.cfg fast path**: If the ISO has a `loopback.cfg`, parse
    it and resolve GRUB variables (`${iso_path}`, `${isofile}`) to extract the
    ISO kernel params from loopback entries.