Enable autofs support explicitly (#4984)

Supervisor pairs every network mount with a systemd .automount unit,
so the kernel automounter is a hard requirement for network storage.

Today the option is only enabled as a side effect: Buildroot's systemd
package force-enables CONFIG_AUTOFS_FS through its kernel config
fixups. That works — it even overrode the explicit disable the
Raspberry Pi config carried until 11.2 — but nothing in our configs
states the dependency, and the fixup skips symbols already set to =m,
which leaves the Rockchip boards (Green, ODROID-M1/M1S) with autofs as
a module while every other board has it built in.

Set the option in the shared fragment to document the requirement and
build it in everywhere.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

buildroot-external/kernel/v6.18.y/haos.config

@@ -209,6 +209,8 @@ CONFIG_CIFS_XATTR=y
 CONFIG_CIFS_POSIX=y
 CONFIG_CIFS_DFS_UPCALL=y
 
+CONFIG_AUTOFS_FS=y
+
 CONFIG_WIREGUARD=m
 
 CONFIG_HID=y