initramfs: use kstrtobool() instead of strtobool()

strtobool() is the same as kstrtobool().  However, the latter is more used
within the kernel.

In order to remove strtobool() and slightly simplify kstrtox.h, switch to
the other function name.

While at it, include the corresponding header file (<linux/kstrtox.h>)

Link: https://lkml.kernel.org/r/2597e80cb7059ec6ad63a01b77d7c944dcc99195.1673716768.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Disseldorp <ddiss@suse.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Martin Wilck <mwilck@suse.com>
Cc: wuchi <wuchi.zero@gmail.com>
Cc: XU pengfei <xupengfei@nfschina.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>