MFC r360271-r360272, r360321

r360271:
Add support for generating release images using GPT for ARM

Submitted by: Daniel Engberg (Original version)
Differential Revision: https://reviews.freebsd.org/D22537

r360272:
Add PINE64 ROCKPro64 config for generation of release images

Submitted by: Daniel Engberg
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D22538

r360321:
release: arm64: rockpro64: Set hw.ncpu to 4

Since there is known issue with big.LITTLE set the number of CPU to 4
which is the number present in the LITTLE cluster.