tools/bootconfig: render kernel.* subtree as cmdline string with -C

tools/bootconfig: render kernel.* subtree as cmdline string with -C

Add a -C option that finds the "kernel" subtree of a bootconfig file
and prints it as a flat, space-separated cmdline string by calling the
shared xbc_snprint_cmdline() renderer. An empty or absent kernel.*
subtree produces empty output and exits successfully.

This lets the kernel build embed a bootconfig file as a plain cmdline
string at build time, so embedded bootconfig values can reach
parse_early_param() during architecture setup without parsing the
bootconfig at runtime.

The renderer is intentionally limited to the kernel.* subtree: that is
the only thing the kernel build needs to embed; init.* and other
subtrees keep going through the runtime parser.

Example of this new mode:
	# cat /tmp/test.bconf
	kernel {
		foo = bar
		baz = "hello world"
		arr = 1, 2
	}
	init.foo = nope

	# ./tools/bootconfig/bootconfig -C /tmp/test.bconf
	foo=bar baz="hello world" arr=1 arr=2 %

Link: https://lore.kernel.org/all/20260508-bootconfig_using_tools-v1-2-1132219aa773@debian.org/
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>