Improve AUR package templates

[relu]

Improvement recommendations from @FabioLolix on AUR: https://aur.archlinux.org/packages/flux-bin#comment-928509

Hello,

• you can remove armv6h since that is has been removed from the distro
• please remove ${pkgname}-${pkgver}.tar.gz:: from source_*=(), renaming all the files to the same name is detrimental if one need to use the command updpkgsums
• improve pkgver/_srcver instead of duplicate it by using for example:

_srcver=2.0.0-rc.5
pkgver=${_srcver//-/.}

or

_srcver=2.0.0-rc.5
pkgver=${_srcver//-/}