Add support for ARM build pipeline

Modify the existing python code to be able to build images where the
image tag can be prepended with a prefix, here arm_. This is preparatory
work to create the ARM build image pipeline, and reuse the existing
internal registry on porcepix to have x86 and ARM images coexist.

Fix various bugs in the build system to allow to build on a system with
as many cores as gracehopper.

Also, fix two programs (usim, nvram) to work correctly under ARM.