Feature Request: Converge riscv on one Dockerfile

[t-fine]

Individual riscv architecture Dockerfiles should be consolidated into the single Dockerfile method recently implemented.

Feature Request: helloworld - Converge on one Dockerfile #560

Riscv example docker files to migrate: https://github.com/open-horizon/examples/blob/master/edge/services/helloworld/Dockerfile.riscv64 https://github.com/open-horizon/examples/blob/master/edge/services/cpu_percent/Dockerfile.riscv64 https://github.com/open-horizon/examples/blob/master/edge/services/gps/Dockerfile.riscv64