Add CPUMiner package - a bitcoin/litecoin CPU miner

As discussed in issue #751, and to fill my needs, here is a CPUminer package.

based on pooler work : https://github.com/pooler/cpuminer

Compilation tested on theses architectures :
- [x] 88f6281
- [x] armada370
- [x] armadaxp
- [x] bromolow
- [x] cedarview
- [x] evansport
- [x] ppc853x
- [x] qoriq
- [x] x86

Package installation tested on a RS3614xs+ with bromolow architecture - works fine for 3 days.

This is my first synology package, feel free to comment and discuss this PR.

I think the correct syntax here is ADDITIONAL_CFLAGS.