linux-yocto/6.17: fix rdinit boot warning

linux-yocto/6.17: fix rdinit boot warning

Integrating the following commit(s) to linux-yocto/6.17:

1/1 [
    Author: Bruce Ashfield
    Email: bruce.ashfield@gmail.com
    Subject: boot: only emit rdinit warning on initramfs boot
    Date: Mon, 27 Oct 2025 09:47:05 -0400

    commit 98aa4d5d242d3a73 [init/main.c: add warning when file specified in
    rdinit is inaccessible] promoted a long time check to be visible on
    boot.

    The issue is that it is always issued even when an initramfs boot is
    not used.

    To avoid needing to completely disable CONFIG_BLK_DEV_INITRD and not
    have the warning issues when an initramfs isn't used, we add checks for
    the existence and size of an initramfs before allowing the warning
    to be generated.

    Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
]

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>