Resolve package dependencies for unattended build.

Dependent packages are automatically downloaded to allow the build to
complete without user intervention.
Use cdn.debian.net instead of http.debian.net.

http.debian.net seems flakier than cdn.debian.net, so use the more
reliable alternative.
Remove uboot-mkimage: it's now u-boot-tools.

uboot-mkimage has been renamed u-boot-tools, which we've already
included in the package list.
CUSTOMIZATIONS variable: arbitrary, exciting, extensiblity.

Treat variable values as system attributes and use that to add new
packages, tools, or options down the line.  Maybe it'll turn out to be
just bad design, but it's sure handy.
Added JTAG connection details.
Fixed mercurial package name.
Added a few more necessary dev packages.
Added instructions to save DP MAC addresses before flashing.

HUGE thanks to Markus for providing the information I needed to
actually figure this whole ridiculous mess out.
Merge from Nick.