Welcome
Typically, the boot configuration files for a USB drive are stored in extlinux.conf file located usually at
/tce/boot/extlinux/extlinux.conf or
/boot/extlinux/extlinux.conf
adding waitusb=5 to the command line is usually sufficient, similar to this
kernel /boot/vmlinuz
append initrd=/boot/core.gz loglevel=3 waitusb=5 tce=UUID="c4d1fd25-16d2-41bf-8de4-2986104f99a4"
note that depending on the installation method the location and file name extlinux.conf might differ slightly, you may find either
isolinux (normally used in CD's), extlinux or syslinux.conf files, expecting extlinux.conf installed to a USB thumb drive however.
You may find that using UUID to identify the USB drive will help the system find the correct device by specifying the UUID along with waitusb=5 like this
waitusb=5:UUID="c4d1fd25-16d2-41bf-8de4-2986104f99a4"
see the wiki for more info
You may also find that adding tce=UUID="c4d1fd25-16d2-41bf-8de4-2986104f99a4" to the same boot config as above will help with storing extensions over reboots, see the persistence section of the wiki for more info
tc-5.0.1 is the current release I believe