rpi: update to latest kernel; fix rootfs size

Updating Buildroot forced the kernel update, but it seems to be
harmless. The rootfs max size was also fixed to be 128 MB like specified in
fwtool.config in the comments.
Add patch to dtach to support redirecting the erl prompt
nerves.mk: alternative to sourcing nerves-env.sh

For Nerves configurations that pull in Nerves as a subdirectory to the
project, this file can be included in a Makefile to achieve similar
results as sourcing nerves-env.sh.
Bump buildroot now that they're getting close to release

Clean up dtach patch