udevd isn't on $PATH and is in different places depending on what version of
udev is being used. Copy the code from initramfs-framework to search for the
right udevd at boot time instead of hard-coding a location.
(From OE-Core rev: a5de76317112fc065a78a745c2f23775dca15d75)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>