[OE-core] Configurable host name

[Burton, Ross]

Hi,

I think that the current hostname setting of $MACHINE (unless you
override base-files and change $hostname there) is too restrictive.
Distributions should be able to override it easily (i.e. adding a
distro-specific prefix) and individuals should also be able to change
it in local.conf (i.e. assigning a real-world name, i.e. beagle or
zbox).

My proposal would be to add something like HOSTNAME ?= "$MACHINE" to
default-distrovars.inc, and changing base-files to write the hostname
file in a new task using something like addtask hostname after
do_rootfs.

This is entirely untested but opinions (especially "you're wrong" or
"there's a better way") welcome.

Ross