Add armhf Debian package to release

[hnw]

Add armhf(32bit ARM with hardfloat) Debian package support.

The package has already been tested in two weeks on my own Raspberry Pi 2, and it works fine.

However, this pull request has one problem. This change requires Docker image yhnw/docker-mackerel-deb-builder. We should use Docker image which is managed by Mackerel dev team.

If the dev team approve mackerelio/docker-mackerel-deb-builder#1, then I'll fix the problem.

[astj]

[memo]

• In Debian "armhf" supports ARMv7~ https://www.debian.org/ports/arm/
  • "armhfp" for CentOS also supports ARMv7~ https://wiki.centos.org/SpecialInterestGroup/AltArch/armhfp
• From those point of view, ARMv7 seems enough, but:
• Raspberry Pi 1 (and zero?) are ARMv6 https://en.wikipedia.org/wiki/Raspberry_Pi#Specifications
• For those ARMv6 family, Raspberry Pi teams maintain Debian fork (Raspbian) for arm with ARMv6 https://www.raspbian.org/RaspbianFAQ#What_is_Raspbian.3F

[astj]

As "armhf" architecture, GOARM=7 is enough specification to build armhf-compatible, since "armhf" Debian supports >= ARMv7.
If you need ARMv6, the reason should be present in somewhere or different name than "armhf" should be used, to avoid confusion.

[astj]

@hnw Thank you for pull request! Adding 32bit arm packages seems good, but maybe we need some discussion around GOARM parameter. Please check the review comment above.

[hnw]

Hi @astj. Thanks for the comment.

I fixed a Docker image and added comments.

As you mentioned above, ARMv6 is called as armhf on Raspbian (it's confusing but might be harmless because of upper compatibility). Raspberry Pi is widely used, so I'd like to support them.

[astj]

Sorry to have kept you waiting so long. LGTM!