build_config: AArch64 (arm64) basic support

build_config: AArch64 (arm64) basic support

- parameterize repo URIs in build_config.ini based on target machine
  architecture where possible;
- add support for diverging values of the same variable based on target
  machine architecture: since nested variable interpolation is not
  supported, implement a mechanism in build-tools which expands
  variables containing the magic string '#ARCH#' to $(uname -m);
- while at it, drop '.x86_64' from packages.yaml versions and let yum
  automatically choose the proper package;

Depends-on: I32cfdc2adc43fb703843383a1e94a807919f4e8c

Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>
Co-authored-by: Valentin Radulescu <Valentin.Radulescu@enea.com>
Change-Id: I3a940e2e0ece6febc9fc90d6bbbc6ff8ef212e14