kernel: Bump to 3.18.1

kernel: DMA: fix cyclic LITE length overflow bug
See: https://github.com/raspberrypi/linux/issues/737

firmware: arm_loader: Fix negative voltage offsets through mailbox interface

firmware: gencmd: Show extra decimal digit on voltage reported

firmware: gencmd: Add gencmd to read boot RSTS (e.g. to check for watchdog reset)

firmware: gencmd: Add memory test gencmd

firmware: DT: Add config.txt device_tree_param mechanism
See: http://www.raspberrypi.org/forums/viewtopic.php?f=29&t=93015