initrdscripts/init-install*: Add rootwait when installing to USB devices

initrdscripts/init-install*: Add rootwait when installing to USB devices

It can take a bit for USB devices to be detected, so if a USB device is
your rootfs and you don't set rootwait you will most likely get a kernel
panic. Fix this by adding rootwait to the kernel command line on
installation.

Fixes [YOCTO #9462].

Signed-off-by: California Sullivan <california.l.sullivan@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>