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.