Allow forcing unconditional bootconfig processing

The BOOT_CONFIG family of Kconfig options allows a bootconfig file
containing kernel boot parameters to be embedded into an initrd or into
the kernel itself.  This can be extremely useful when deploying kernels
in cases where some of the boot parameters depend on the kernel version
rather than on the server hardware, firmware, or workload.

Unfortunately, the "bootconfig" kernel parameter must be specified in
order to cause the kernel to look for the embedded bootconfig file,
and it clearly does not help to embed this "bootconfig" kernel parameter
into that file.

Therefore, provide a new BOOT_CONFIG_FORCE Kconfig option that causes the
kernel to act as if the "bootconfig" kernel parameter had been specified.
In other words, kernels built with CONFIG_BOOT_CONFIG_FORCE=y will look
for the embedded bootconfig file even when the "bootconfig" kernel
parameter is omitted.  This permits kernel-version-dependent kernel
boot parameters to be embedded into the kernel image without the need to
(for example) update large numbers of boot loaders.

Link: https://lore.kernel.org/all/20230105005838.GA1772817@paulmck-ThinkPad-P17-Gen-1/

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <linux-doc@vger.kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>