usb_drv: Wait for IRQ on SKB exhaustion

Do not use 'wait_event' when SKB-backend allocation fails, explicitly wait for
an interrupt, which in turn frees SKBs, instead.

Fixes issue #788
Check md5 sum of initramfs file in l4lx run-script

* Always download md5sum file and check downloaded binaries
* Extend sleep time in l4linux_netperf script, so that DHCP has enough time
* Fix regexp rule in l4linux_netperf run script to work with MAC addresses
  containing only numbers

Fix #787