Improve POAP via USB experience

[mib1185]

This fixes the default path variables when using POAP via USB, so the root directory gets used.

Prior this change, it tries to use /usbslot1//var/lib/tftpboot/ as config and target image path

INFO: Starting Copy of Config File to /bootflash/poap_conf.cfg - /script.sh
Copying file options source=/var/lib/tftpboot/conf.SNSNSNSNSN destination=/bootflash/poap_conf.cfg login_timeout=120 destination_tmp=poap_conf.cfg.tmp - /script.sh
/usbslot1//var/lib/tftpboot/conf.SNSNSNSNSN does NOT exist - /script.sh

INFO: Starting Copy of System Image - /script.sh
Copying file options source=/var/lib/tftpboot/nxos64-cs.10.4.4.M.bin destination=/bootflash/nxos64-cs.10.4.4.M.bin login_timeout=2100 destination_tmp=nxos64-cs.10.4.4.M.bin.tmp - /script.sh
/usbslot1//var/lib/tftpboot/nxos64-cs.10.4.4.M.bin does NOT exist - /script.sh
Cleanup all files - /script.sh